Nitride semiconductor laser device having current blocking layer and method of manufacturing the same

ABSTRACT

A nitride semiconductor laser including a laminate that includes an n-side semiconductor layer, an active layer and a p-side semiconductor layer, the n-side semiconductor layer or p-side semiconductor layer including a current blocking layer  30  that is made of In x Al y Ga 1-x-y N (0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1) and has a stripe-shaped window  32  formed therein to pass current flow.

This application is a divisional of application Ser. No. 10/876,695,filed Jun. 28, 2004 now U.S. Pat. No. 7,227,879.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor laser devicehaving an active layer made of gallium nitride semiconductor,particularly to a nitride semiconductor laser device having a currentblocking layer made of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1, 0.5≦y≦1,0.5≦x+y≦1).

2. Prior Art

Gallium nitride semiconductor laser is capable of oscillating in a widerange of wavelengths from ultraviolet to red light, and is expected tohave variety of applications such as light sources for optical disksystem, laser printer and optical network. In the gallium nitridesemiconductor laser of the prior art, it has been a common practice toemploy ridge waveguide structure that has stripe-shaped ridge formed ona cladding layer or the like located on an active layer, for the stripestructure formed to control the horizontal transverse oscillation mode.

However, since mechanical strength of the ridge waveguide structure isweak at the ridge, defects are likely to occur particularly when mountedface down. Also because variations are caused in the threshold currentand/or beam shape depending on the dimensions of the ridge, it isdifficult to manufacture laser devices of uniform characteristics. Forthis reason, attempts have been made to control the horizontaltransverse oscillation mode by forming an insulation layer (currentblocking layer) having a stripe-shaped window as a current path, overthe active layer, instead of the ridge waveguide structure.

For example, Japanese Unexamined Patent Publication (Kokai) No.2002-314203 proposes a gallium nitride semiconductor laser having acurrent blocking layer formed from AlN in a p-type optical guide layerof the active layer. The stripe structure of this laser is made asfollows. First, the current blocking layer made of AlN is formed on adevice, on which layers up to the p-type optical guide layer have beenformed, at a temperature from 400 to 600° C. in a reaction furnace ofMOCVD apparatus. After taking out the wafer from the reaction furnace,stripe-shaped window is formed by photolithography process using analkaline etching solution. Then the wafer is returned into the reactionfurnace of the MOCVD apparatus where p-type optical guide layer is grownso as to fill in the window of the current blocking layer, and p-typecladding layer and other layers are formed successively.

SUMMARY OF THE INVENTION

In the gallium nitride semiconductor laser described above, however, thewafer must be taken out of the reaction furnace of the MOCVD apparatusin order to carry out the process of forming the stripe-shaped window inthe current blocking layer. Since the wafer taken out of the reactionfurnace is exposed to the ambient atmosphere such as air, an oxide layeror the like is formed on the surface of the semiconductor layer throughreaction with the atmosphere. Existence of such a layer leads to lowerperformance of the device, and therefore an operation to remove thelayer by etching (hereafter called the etch-back) must be carried outwhen the wafer is returned to the MOCVD apparatus and semiconductor isgrown again. The etch-back operation is usually carried out by blowinghydrogen gas, that is a reducing gas, onto the wafer which is kept at ahigh temperature in the reaction furnace.

However, since the thickness and constitution of the layer formed on thesurface of the semiconductor layer through reaction with the atmospherevary among wafers and among chips that are formed on a wafer, it isdifficult to accurately remove only the layer on a stable basis. Whenpart of the layer remains due to insufficient etch-back on the interfaceof re-growth, device characteristics will be compromised. Especiallywhen the layer remains in the window of the current blocking layer, theremaining layer makes uneven distribution of the current, thus resultingin uneven light emission. When the etch-back proceeds excessively, onthe other hand, not only the layer formed through reaction with theatmosphere but also the semiconductor layer located thereunder (forexample, the p-type optical guide layer in the case of PatentDocument 1) is etched. If re-growth is carried out under this condition,core of the waveguide becomes too thin in case the underlyingsemiconductor layer is the optical guide layer, disabling it to confinelight satisfactorily. Also because the step in the window becomes toolarge by over etching, composition of the semiconductor formed byre-growing becomes inhomogeneous due to the step, thus adverselyaffecting the device characteristics.

With the background described above, it is an object of the presentinvention to provide a gallium nitride semiconductor laser that has acurrent blocking layer provided with a stripe-shaped window and has adevice structure that ensures stable characteristics, and a method ofmanufacturing the same.

According to the present invention, there is provided a nitridesemiconductor laser comprising a laminate that includes an n-sidesemiconductor layer, an active layer and a p-side semiconductor layer,said n-side semiconductor layer or p-side semiconductor layer includinga current blocking layer that is made of In_(x)Al_(y)Ga_(1-x-y)N(0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1) and has a stripe-shaped window formedtherein to pass current flow, wherein said current blocking layer isformed on a semiconductor layer having less Al ratio than said currentblocking layer, and said semiconductor layer is removed from the portioncorresponding to the window of said current blocking layer.

More specifically, according to one aspect of the present invention,there is provided a nitride semiconductor laser device composed of alaminate comprising an n-side semiconductor layer, an active layer and ap-side semiconductor layer, said laminate having a current blockinglayer that is made of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1, 0.5≦y≦1,0.5≦x+y≦1) and has a stripe-shaped window formed therein, and asemiconductor layer is formed on the current blocking layer and thewindow, wherein the current blocking layer is formed on (a) a firstsemiconductor layer containing Al and (b) a second semiconductor layerthat is formed on the first semiconductor layer and does not contain Alor has a lower mixed crystal ratio of Al than that of the currentblocking layer, while the second semiconductor layer is partiallyremoved in a portion corresponding to the window of the current blockinglayer.

Also according to the first aspect of the present invention, there isprovided a method of manufacturing a nitride semiconductor laser devicecomposed of a laminate comprising an n-side semiconductor layer, anactive layer and a p-side semiconductor layer, said laminate having acurrent blocking layer that is made of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1,0.5≦y≦1, 0.5≦x+y≦1) and has a stripe-shaped window formed therein, and asemiconductor layer is formed on the current blocking layer and thewindow, the method comprising the steps of:

(a) forming a first semiconductor layer made ofIn_(x1)Al_(y1)Ga_(1-x1-y1)N (0≦x₁≦0.1, 0.1≦y₁≦1, 0.1≦x₁+y₁≦1) on p sideor n side of the active layer;

(b) forming a second semiconductor layer made ofIn_(x2)Al_(y2)Ga_(1-x2-y2)N (0≦x₂≦1, 0≦y₂≦0.1, 0≦x₂+y₂≦1) where a mixedcrystal ratio of Al, y₂, satisfies the expression y₂<y₁ and y₂<y₃ on thefirst semiconductor layer;

(c) forming a current blocking layer made of In_(x3)Al_(y3)Ga_(1-x3-y3)N(0≦x₃≦0.1, 0.5≦y₃≦1, 0.5≦x₃+y₃≦1) on the second semiconductor layer;

(d) forming a stripe-shaped window by removing a part of the currentblocking layer to such a depth that reaches the second semiconductorlayer; and

(e) removing the second semiconductor layer that is exposed through thecurrent blocking layer to such a depth that reaches the firstsemiconductor layer.

According to the first aspect of this invention, stable lasercharacteristics can be obtained as the layer formed through reactionwith the atmosphere is prevented from remaining in the window of thecurrent blocking layer and defective shape can be prevented from beingformed due to excessive etch-back, by forming the first and secondsemiconductor layers below the current blocking layer.

Specifically, since the second semiconductor layer is made of a nitridesemiconductor that does not contain Al or has a lower mixed crystalratio of Al than that of the current blocking layer, it serves as anetching stopper layer when forming the window in the current blockinglayer and also protects the device layer located thereunder from ambientgas such as oxygen, before eventually being removed by etch-back processcarried out in a vapor phase growth apparatus.

It is preferable that the second semiconductor layer has a mixed crystalratio of Al lower than that of the first semiconductor layer. Thiscauses the second semiconductor layer to be etched back at a faster ratethan the first semiconductor layer that makes contact with the bottomthereof during the etch-back carried out in the vapor phase growthapparatus. As a result, stable laser characteristics can be obtained asthe layer formed through reaction with the atmosphere is prevented fromremaining in the window of the current blocking layer and defectiveshape can be prevented from being formed due to excessive etch-back. Atthis time, the first semiconductor layer serves as an etching stopperlayer during the etch-back process and also protects the device layerlocated thereunder from gas etching.

According to a second aspect of the present invention, there is provideda nitride semiconductor laser device composed of a laminate comprisingan n-side semiconductor layer, an active layer and a p-sidesemiconductor layer, said laminate having a current blocking layer thatis made of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1) and hasa stripe-shaped window formed therein, and a semiconductor layer isformed on the current blocking layer and the window, wherein the currentblocking layer is formed on a growth base layer that is made ofsemiconductor having a lower mixed crystal ratio of Al than that of thecurrent blocking layer, the growth base layer preferably being such thatdecomposes at a lower temperature than the current blocking layer doesand is partially removed in a portion thereof corresponding to thewindow of the current blocking layer.

Also according to the second aspect of the present invention, there isprovided a method of manufacturing a nitride semiconductor laser devicecomposed of a laminate comprising an n-side semiconductor layer, anactive layer and a p-side semiconductor layer, said laminate having acurrent blocking layer that is made of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1,0.5≦y≦1, 0.5≦x+y≦1) and has a stripe-shaped window formed therein, and asemiconductor layer is formed on the current blocking layer and thewindow, the method comprising the steps of:

(a) forming a growth base layer made of In_(x′)Al_(y′)Ga_(1-x′-y′)N(0≦x′≦1, 0≦y′≦0.1, 0≦x′+y′≦1) on p side or n side of the active layer;

(b) forming a current blocking layer made of In_(x)Al_(y)Ga_(1-x-y)N(0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1) on the growth base layer;

(c) forming a stripe-shaped window by removing a part of the currentblocking layer to such a depth that reaches the growth base layer;

(d) removing the growth base layer that is exposed through the currentblocking layer to such a depth as the layer that makes contact with thebottom of the growth base layer is exposed.

According to the second aspect of this invention, stable lasercharacteristics can be obtained as the layer formed through reactionwith the atmosphere is prevented from remaining in the window of thecurrent blocking layer and defective shape can be prevented from beingformed due to excessive etch-back, by forming only the growth base layerof lower Al ratio below the current blocking layer and making thecrystallinity of the growth base layer lower than that of the layer thatmakes contact with the bottom of the growth base layer.

Specifically, since the growth base layer is made of a nitridesemiconductor that has lower Al ratio than that of the current blockinglayer, it serves as an etching stopper layer when forming the window inthe current blocking layer and also protects the device layer locatedthereunder from ambient gas such as oxygen, before eventually beingremoved by etch-back process carried out in the vapor phase growthapparatus. Also because the growth base layer is formed with lowercrystallinity than that of the layer that makes contact with the bottomthereof, it is removed at a faster rate than the layer locatedthereunder in the etch-back. As a result, stable laser characteristicscan be obtained as the layer formed through reaction with the atmosphereis prevented from remaining and excessive etch-back is prevented.

According to a third aspect of the present invention, there is provideda nitride semiconductor laser device composed of a laminate comprisingan n-side semiconductor layer, an active layer and a p-sidesemiconductor layer, said laminate having a current blocking layer thatis preferably made of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1, 0.5≦y≦1,0.5≦x+y≦1) and has a stripe-shaped window formed therein, wherein thereis a residual film portion where part of the current blocking layer thatmakes contact with the base layer remains in the stripe-shaped window,so that current can be injected into the active layer through theresidual film portion.

Also according to the third aspect of the present invention, there isprovided a method of manufacturing a nitride semiconductor laser device,the method comprising the steps of:

(a) forming a current blocking layer made of In_(x)Al_(y)Ga_(1-x-y)N(0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1) on p side or n side of the active layer;

(b) forming a stripe-shaped window by removing a part of the currentblocking layer so that part of the current blocking layer that makescontact with the base layer remains; and

(c) etching the surface of the current blocking layer.

The present inventors found that the current blocking layer made of anitride semiconductor that contains Al in a high concentration such asAlN has a high insulating property but there is a portion where currentcan flow easily in the vicinity the base layer whereon the currentblocking layer grows. This is supposedly because the current blockinglayer inherits a crystallinity from the base layer thereunder, resultingin high crystallinity in the portion of the current blocking layer nearthe base layer. The base layer that makes contact with the bottom of thecurrent blocking layer has crystallinity becoming higher gradually fromthe interface with the substrate upward due to continuous growth. Whenthe current blocking layer made of an insulating material such as AlN isgrown thereon, the portion formed in the early stage of growing has highcrystallinity so as to allow current to flow relatively easily therein.Also in the portion of the current blocking layer formed in the earlystage of growing, a trace of impurities can easily mix which also makesa cause of allowing current to flow. When the current blocking layer iscaused to continue growing, on the other hand, resistivity increases soas to become more insulating as the growth proceeds, since the nitridesemiconductor having a high mixed crystal ratio of Al has a tendency tohave lower crystallinity as it grows. The third invention makes use ofsuch a property, and eliminates the possibility of blocking satisfactorycurrent injection into the active layer by leaving a proper thickness ofthe current blocking layer in the portion thereof formed in the earlystage of growing.

The residual film portion of the current blocking layer that is left toremain in the window has high Al ratio and good crystallinity, and istherefore decomposed at a slower rate in reducing atmosphere such ashydrogen gas. Therefore, such a portion left in the window can serve asan etching stopper layer when removing the layer formed through reactionwith the atmosphere from the wafer surface by etch-back. Although theremaining portion of the current blocking layer in the window is alsodecomposed a little, sufficient function of the etching stopper layercan be achieved if the thickness of the remaining portion of the currentblocking layer is properly set such that the layer somewhat remainsafter decomposition. If the remaining portion of the current blockinglayer functions as the etching stopper layer, stable lasercharacteristics can be obtained by preventing the layer formed throughreaction with the atmosphere from remaining on the wafer surface andexcessive etch-back from occurring.

The part of the current blocking layer left to remain also has an effectof mitigating the step in the window. Since the present inventionutilizes the portion of the current blocking layer, that has goodcrystallinity and has been observed in the current blocking layer of theprior art, as the etching stopper layer, total thickness of the currentblocking layer may be comparable to that of the prior art. As aconsequence, the part of the current blocking layer left to remain inthe window decreases the height of the step in the window, makes thecomposition of the layer formed thereon more uniform, and mitigatesother problems originating from the step.

Thickness of the portion of the layer remaining after etch-back ispreferably 10 Å or more in order to function as the etching stopperlayer and decrease the step in the window. Thickness of the residualfilm portion of the current blocking layer is also preferably less than100 Å, since current injection into the active layer is hampered when itis too thick.

Thus according to the third invention, stable laser characteristics canbe obtained by leaving a portion of the current blocking layer toremain, made of nitride semiconductor of high Al content, that islocated near the base layer. The remaining portion has goodcrystallinity so as to prevent the layer formed through reaction withthe atmosphere from remaining on the wafer surface and excessiveetch-back from occurring.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view showing a gallium nitride semiconductor laserdevice according to first embodiment of the invention.

FIGS. 2A and 2B are partially enlarged sectional views showing thegallium nitride semiconductor laser device according to the firstembodiment of the invention.

FIG. 3 is a sectional view showing a gallium nitride semiconductor laserdevice of the prior art.

FIG. 4A through 4D are process diagrams showing a method ofmanufacturing the gallium nitride semiconductor laser device accordingto the first embodiment.

FIG. 5 is a plan view showing an area where current blocking layer is tobe formed in the gallium nitride semiconductor laser according to thefirst embodiment of the invention.

FIG. 6 is a sectional view showing a gallium nitride semiconductor laseraccording to second embodiment of the invention.

FIG. 7 is a sectional view showing a gallium nitride semiconductor laserdevice according to third embodiment of the invention.

FIG. 8A through 8D are process diagrams showing a method ofmanufacturing the gallium nitride semiconductor laser device accordingto the third embodiment.

FIG. 9 is a sectional view showing a gallium nitride semiconductor laseraccording to fourth embodiment of the invention.

FIG. 10 is a sectional view showing a gallium nitride semiconductorlaser device according to fifth embodiment of the invention.

FIGS. 11A and 11B are partially enlarged sectional views showing thegallium nitride semiconductor laser device according to the fifthembodiment.

FIG. 12A through 12D are process diagrams showing a method ofmanufacturing the gallium nitride semiconductor laser device accordingto the fifth embodiment.

FIG. 13 is a sectional view showing a gallium nitride semiconductorlaser according to sixth embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now preferred embodiments of the gallium nitride semiconductor laseraccording to the present invention will be described with reference tothe accompanying drawings. In the drawings, identical numerals denotethe same or corresponding members.

In this specification, bottom or lower side of the gallium nitridesemiconductor laser refers to the side of the semiconductor layer thatconstitutes the laser where it started to grow, and top or upper siderefers to the side of the semiconductor layer where it stopped growing.Since the growing direction of the semiconductor layer substantiallyagrees with the direction in which dislocation proceeds, starting pointof dislocation corresponds to the bottom or lower side and ending pointof dislocation corresponds to the top or upper side in the laser device.

In this specification, the notion that the gallium nitride semiconductorhas good crystallinity means that density of etch pits is relatively lowwhen measured in wet etching, or that the layer is relatively difficultto remove by wet etching.

Embodiment 1

FIG. 1 is a sectional view showing a gallium nitride semiconductor laserdevice according to this embodiment. An n-side contact layer 4 made ofGaN, an n-side cladding layer 6 made of AlGaN, an n-side optical guidelayer 8 made of GaN, a multiple quantum well active layer 10 having awell layer containing In, a p-side optical guide layer 12 made of GaN, ap-side cladding layer 14 made of AlGaN and a p-side contact layer 16made of GaN are formed on a substrate 2 made of a different materialsuch as sapphire. Formed in the p-side optical guide layer 12 is acurrent blocking layer 30 that has a stripe-shaped window 32. Thecurrent blocking layer 30 is made of a gallium nitride semiconductorthat contains Al in ratio of 0.5 or higher and has high resistivity, soas to concentrate the current within the active layer 10 located in thewindow 32 and control horizontal transverse mode of laser oscillation.

FIGS. 2A and 2B are partially enlarged sectional views showing thestructure in the vicinity of the current blocking layer 30 in moredetail. As shown in FIG. 2A, a carrier confinement layer 11 made ofgallium nitride semiconductor containing Al is formed with a thicknessfrom 50 to 150 Å on the active layer 10 made of gallium nitridesemiconductor containing In, and the p-side optical guide layer 12 madeof GaN is formed thereon. The p-side optical guide layer 12 comprises afirst p-side optical guide layer 12 a located below the current blockinglayer 30 and a second p-side optical guide layer 12 b. The currentblocking layer is formed above the first p-side optical guide layer 12 avia a first semiconductor layer 22 and a second semiconductor layer 24,and the window 32 is formed to penetrate the current blocking layer 30and the second semiconductor layer 24. The second p-side optical guidelayer 12 b is formed so as to fill in the window 32.

The current blocking layer 30 is not only high in resistance but alsolow in crystallinity because of the high Al ratio of 0.5 or higher. As aresult, as shown schematically in FIG. 2A, dislocations 40 occur with ahigh density also in the p-side cladding layer 14 and the p-side contactlayer 16 formed above the current blocking layer 30, thus making itdifficult for current to flow therein. That is, the current blockinglayer 30 exercises current block effect in the semiconductor layerlocated above it by decreasing the crystallinity thereof, in addition tothe current block effect achieved by the resistance of itself.Therefore, even when the current blocking layer 30 is formed with arelatively small thickness of 100 to 500 Å, it can perform sufficientcurrent blocking effect through the combined effect of high resistanceand low crystallinity of itself.

The second semiconductor layer 24 located below the current blockinglayer 30 is made of a nitride semiconductor having a mixed crystal ratioof Al lower than that of the current blocking layer 30, so as tofunction as an etching stopper layer when the window 32 is formed in thecurrent blocking layer 30 by photolithography, and also protect thedevice layer from ambient gas such as oxygen, before eventually beingremoved from the current path by etch-back carried in a vapor phasegrowth apparatus.

This is because the second semiconductor layer 24 has The Al ratio(preferably 0.1 or less) lower than that of the current blocking layer30, it is etched with an alkaline solution at a rate different from thatof the current blocking layer 30 that has the Al ratio of 0.5 or higher,and therefore remains without being etched when the current blockinglayer 30 is etched with an alkaline solution. Thus when the window 32 isformed in the current blocking layer 30, the second semiconductor layer24 serves as the etching stopper layer and prevents excessive etching.Also because the second semiconductor layer 24 is made of a nitridesemiconductor having low Al ratio, it reacts at a low rate with oxygenand other elements contained in air. Therefore, the second semiconductorlayer 24 can effectively protect the first semiconductor layer locatedthereunder from the ambient gas such as oxygen in the photolithographyprocess carried out outside the vapor phase growth apparatus.

In the meantime, the second semiconductor layer 24 is damaged on thesurface thereof through the etching process to remove the currentblocking layer 30 and exposure to the atmosphere. However, because thesecond semiconductor layer of this embodiment is made of a nitridesemiconductor having lower Al ratio that that of the first semiconductor22, it easily decomposes when exposed to a reducing gas such as hydrogenat a high temperature. As a result, the second semiconductor layer 24that has been damaged can be easily removed from the window 32 thatbecomes the current path, by the etch-back process carried out in thevapor phase growth apparatus.

The first semiconductor layer 22, on the other hand, serves as anetching stopper layer during the etch-back process carried out beforere-growth on the current blocking layer 30 in the vapor phase growthapparatus and also protects the first p-side optical guide layer 12 alocated thereunder from gas etching. The first semiconductor layer 22 ismade of gallium nitride semiconductor having higher Al ratio than thesecond semiconductor layer and is not easily decomposed when exposed toreducing gas such as hydrogen at a high temperature. Therefore, evenwhen the etch-back process is carried out in the vapor phase growthapparatus for a long period of time so as to completely remove thedamaged second semiconductor layer 24, the etch-back process stops atthe first semiconductor layer 22 and the first p-side optical guidelayer 12 a is protected from excessive etching.

While the first semiconductor layer 22 remains in the current path thatleads to the active layer 10, it has better crystallinity and lowerresistance than the current blocking layer 30 because it has Al ratiocomparable to or lower than that of the current blocking layer and ispreferably grown at a higher temperature than that of growing thecurrent blocking layer. Also because it suffices that the firstsemiconductor layer 22 has the minimum thickness required to function asthe etching stopper layer during etch-back, it may be formed in such athin film that does not block the current injection into the activelayer 10, and substantially no increase occurs in the threshold currentof laser.

As described above, with the gallium nitride semiconductor laseraccording to this embodiment, due to the complementary actions of thefirst semiconductor layer and the second semiconductor layer, the layerformed through reaction with the atmosphere is prevented from remainingin the window of the current blocking layer 30 and defective shape canbe prevented from being formed due to excessive etch-back, thusachieving stable laser characteristics.

Also because excessive etching is prevented by the first and secondsemiconductor layers, flatness of the layer formed on the currentblocking layer 30 and the device characteristics are improved as shownin FIG. 2A. Low crystallinity of the current blocking layer 30 alsocontributes to the improvement in flatness. That is, crystal grows at afaster rate in a region 36 located above the window 32 than in a region38 located above the current blocking layer 30, since the currentblocking layer 30 has lower crystallinity. As a result, as shown in FIG.2A, the window 32 which is a recess can be easily filled so as to form aflat surface with the second p-side optical guide layer 12 b. As thesecond p-side optical guide layer 12 b is formed flat, compositions ofthe p-side cladding layer 14 and the p-side contact layer 16 formedthereon can be restricted from becoming inhomogeneous, so that thefunctions of the layers are improved. In case the p-side cladding layer14 has super lattice structure, in particular, it is important that thesecond p-side optical guide layer 12 b fills the window 32 to make aflat surface. Because the super lattice structure is disturbed whenthere is a step on the surface of the p-side optical guide layer 12 thatmakes contact with the bottom thereof.

In addition, as shown in FIG. 2B, the region 36 located above the window32 may be formed thicker than the region 38 located above the currentblocking layer 30. That is, since the current blocking layer 30 has lowcrystallinity, the region 36 located above the window 32 grows fasterthan the region 38 located above the current blocking layer 30 andtherefore the larger the difference in the rate of growth, the thickerbecomes the region 36 located above the window 32. It is advantageousfor confining light within the waveguide to make the region 36 locatedabove the window 32 thicker. Thickness distribution can be controlled asshown in FIG. 2A or 2B by regulating the crystallinity of the currentblocking layer 30. Crystallinity of the current blocking layer 30 can becontrolled by regulating the Al ratio or the growing temperature of thecurrent blocking layer 30. The higher the Al ratio of the currentblocking layer 30 and the lower the growing temperature, the lowerbecomes the crystallinity of the current blocking layer 30.

In case the structure shown in FIG. 2 does not include the first andsecond semiconductor layers, on the other hand, such troubles becomemore likely to occur as the layer formed through reaction with theatmosphere remains on the first p-side optical guide layer 12 a andetch-back proceeds excessively. FIG. 3 is a sectional view showing thestructure of the first p-side optical guide layer 12 a that has beenetched back excessively. In this embodiment, the first p-side opticalguide layer 12 a is made of GaN and is therefore easily decomposed whenexposed to reducing gas such as hydrogen at a high temperature. As aconsequence, once the etch-back proceeds excessively, the etch-back caneasily proceed up to the carrier confinement layer 11 that is made ofnitride semiconductor containing Al, as shown in FIG. 3. Since the totalthickness of the p-side optical guide layer 12 is usually from 1500 to2000 Å, excessive etch-back of 750 to 1000 Å can occur. As a result, incontrary to the case shown in FIG. 2B, core of the waveguide in theregion 36 that makes the current path becomes thinner compared to thesurrounding region 38, and the efficiency of light confinementdecreases. Also because a large step is made at the end of the currentblocking layer 30, inhomogeneous composition tends to occur such asprecipitation of Al into the step. In case the p-type cladding layer 14has super lattice structure, normal super lattice structure may not bemaintained due to the influence of the step.

Now preferable thickness and composition of each layer will be describedin detail.

[First Semiconductor Layer]

The first semiconductor layer 22 is made of gallium nitridesemiconductor having The Al ratio that is comparable to or lower thanthat of the current blocking layer 30 and is higher than that of thesecond semiconductor layer 24. As such, when compositions of the firstsemiconductor layer, the second first semiconductor layer and thecurrent blocking layer are represented as In_(x1)Al_(y1)Ga_(1-x1-y1)N,In_(x2)Al_(y2)Ga_(1-x2-y2)N and In_(x3)Al_(y3)Ga_(1-x3-y3)N, it ispreferable that the ratios are such that relation y₂<y₁≦y₃ is satisfied.Durability of the first semiconductor layer 22 against the reducing gasbecomes higher as the Al ratio becomes higher. Therefore, the Al ratioy₁ of the first semiconductor layer is preferably 0.1 or higher, andmore preferably 0.2 or higher. When the Al ratio y₁ is too high, on theother hand, resistance of the first semiconductor layer tends to becomehigher. Since the first semiconductor layer 22 becomes a part of thecurrent path to the active layer, higher resistance of the firstsemiconductor layer 22 increases the threshold current of laseroscillation and is not desirable. Thus the Al ratio y₁ of the firstsemiconductor layer is preferably 0.8 or lower, more preferably 0.5 orlower.

The ratio of In contained in the first semiconductor layer 22 ispreferably low. This is because the first semiconductor layer absorbslight emitted from the active layer when it contains In, since the firstsemiconductor layer 22 is located in the waveguide. Thus the firstsemiconductor layer has an In ratio x1 of 0.1 or lower, and preferably0.05 or lower, and more preferably the first semiconductor layeressentially does not contain In. From the discussion described above,preferable composition of the first semiconductor layer isAl_(a)Ga_(1-a)N (0.1≦a≦1).

The first semiconductor layer cannot perform the etching stopperfunction sufficiently during etch-back when it is too thin, and hashigher resistance when it is too thick. Therefore, the thickness of thefirst semiconductor layer is preferably from 20 to 300 Å, and morepreferably from 50 to 200 Å. In case the first semiconductor layer ismade of AlN, it can function as the etching stopper layer when thethickness is about 10 Å or more.

The layer that makes contact with the bottom of the first semiconductorlayer is preferably an optical guide layer. It becomes easier to controlthe confinement of light when the current blocking structure is formedso as to make contact with the optical guide layer.

Function of the first semiconductor layer may also be carried out by acap layer to be described later. When the cap layer is caused to performthe etching stopper function, layers having high Al ratio can bedecreased and therefore the operating voltage can be decreased.

The second semiconductor layer 24 preferably is made of a galliumnitride semiconductor having lower Al ratio than those of the currentblocking layer 30 and the first semiconductor layer 22, namely a galliumnitride semiconductor having a composition represented by the generalformula In_(x2)Al_(y2)Ga_(1-x2-y2)N (0≦x₂≦1, 0≦y₂<y₁, 0≦x₂+y₂≦1). Thelower the Al ratio in the second semiconductor layer, the greater thedifference in the etching rate between the second semiconductor layerand the current blocking layer 30, thus making it easier to remove byetch-back. The second semiconductor layer 24 has the Al ratio y₂ of 0.1or lower, and preferably 0.05 or lower, and more preferably essentiallydoes not contain Al.

The second semiconductor layer preferably contains In. When InN havinglower decomposition temperature is contained in the mixed crystal, thelayer becomes easier to decompose at a high temperature and can beremoved by etch-back more easily. In contained in the secondsemiconductor layer also has an effect of absorbing stray light that hasleaked from the active layer. In other words, light emitted by theactive layer that is made of a nitride semiconductor containing In islikely to be absorbed by another nitride semiconductor containing In.Therefore, by making such a constitution as the second semiconductorlayer that sandwiches the active region on both sides contain In, straylight that has leaked from the active region can be absorbed and qualityof the beam can be improved. With this regard, In ratio x₂ of the secondsemiconductor layer is preferably in a range from 0 to 0.2, and morepreferably from 0.05 to 0.15. From the discussion described above,preferable composition of the second semiconductor layer isIn_(b)Ga_(1-b)N (0.05≦b≦0.15).

The second semiconductor layer is preferably grown with lowcrystallinity in order to make it easy to remove by etch-back.Preferably, the second semiconductor layer is formed in polycrystallineor amorphous structure. Since the second semiconductor layer of such astructure has a high resistance, combined effect thereof and the currentblocking layer formed thereon makes it possible to achieve bettercurrent blocking effect. Also because the second semiconductor layerperforms part of the current blocking effect, the current blocking layermay be formed with a smaller thickness. Crystallinity of the secondsemiconductor layer 24 can be made lower by increasing the In ratio x₂of the second semiconductor layer or decreasing the growing temperature.When the growing temperature of the second semiconductor layer 24 isdecreased, it is preferable to set the growing temperature lower than1000° C., more preferably to 600° C. or lower.

When crystallinity of the second semiconductor layer becomes lower, ithas an effect of making it easier to remove the current blocking layer30 that is grown thereon. That is, if the layer that contacts the bottomof the current blocking layer 30 has good crystallinity, crystallinityof the current blocking layer 30 becomes partially higher in thevicinity of the layer boundary, thus making it difficult to remove theportion by etching. By making crystallinity of the second semiconductorlayer 24 that makes contact with the bottom of the current blockinglayer 30 lower, it is made possible to grow the current blocking layer30 with low crystallinity from the start of growth, thereby making iteasier to remove the current blocking layer 30 in the window 32.Especially when the second semiconductor layer 24 contains impurity in aconcentration of 5×10¹⁷/cm³ or higher, more preferably 5×10¹⁸/cm³ orhigher, crystallinity of the second semiconductor layer 24 becomeslower, thereby making it easier to remove the current blocking layer 30.Also as the second semiconductor layer 24 contains impurities in highconcentration, it becomes easier to absorb stray light coming from theactive layer, so that oscillation of higher mode can be suppressed andlaser beam of stable single mode can be obtained.

The second semiconductor layer 24, if too thin, cannot protect the firstsemiconductor layer 22 sufficiently and, if too thick, makes theinfluence of the step dominant. As the step formed by the currentblocking layer and the second semiconductor layer becomes larger, itbecomes difficult to form the cladding layer and the contact layerlocated above the step in super lattice (SL) structure, resulting inlower carrier mobility and higher operating voltage. Also because itmakes Al, Mg and other elements more likely to precipitate in the step,band gap increases which also contributes to the higher operatingvoltage. Since higher operating voltage leads to larger electric powersupplied, it results in larger heat generation and higher threshold.Based on these considerations, the thickness of the second semiconductorlayer is preferably from 10 to 300 Å, and more preferably from 50 to 200Å.

[Current Blocking Layer 30]

The current blocking layer 30 is made of a nitride semiconductor havinga composition represented by the general formulaIn_(x3)Al_(y3)Ga_(1-x3-y3)N (0≦x₃≦0.1, 0.5≦y₃≦1, 0.5≦x₃+y₃≦1). Formingthe current blocking layer 30 from the gallium nitride semiconductorrepresented by the above general formula, instead of an insulatingmaterial such as SiO₂, makes it possible to grow the current blockinglayer 30 in the same vapor phase growth apparatus as that for the otherdevice structure. Forming the current blocking layer 30 from the galliumnitride semiconductor instead of a different material such as SiO₂ alsohas an effect of improving the linearity of the laser beam. If SiO₂ isembedded in an optical guide layer or the like made of GaN, for example,significant difference between the refractive index of SiO₂, about 1.5,and the refractive index of GaN, that is 2.5, causes a deviation of thelaser output from linearity and the beam to fluctuate. Although the beammay be stabilized by making the current path narrower, it results inhigher current density and shorter service life. If AlN is used for thecurrent blocking layer, in contrast, there is a smaller difference inrefractive index which is 2.1 for AlN and 2.5 for GaN, thus thelinearity is improved and the beam is stabilized.

The higher the Al ratio y₃ in the current blocking layer 30, the higherbecomes the insulating property of the current blocking layer 30, andthe lower becomes the crystallinity of the layer formed thereon, andtherefore the better becomes the current blocking effect. Thus the Alratio y₃ in the current blocking layer 30 is at least 0.5 or higher,preferably 0.75 or higher, and more preferably 0.9 or higher. Mostpreferably, the current blocking layer 30 is made of AlN. When thecurrent blocking layer 30 is made of AlN, such effects are obtained thatwet etching becomes easier, current blocking effect becomes remarkabledue to higher insulating property, light is effectively confined due tolower refractive index and heat dissipation characteristic from thedevice improves due to higher heat conductivity.

The current blocking layer 30 may contain a small amount of In. Makingthe current blocking layer 30 containing a small amount of In makes iteasier to absorb light emitted by the active layer 10. Putting thecurrent blocking layer 30 having such a constitution on the activeregion makes it possible to absorb stray light leaking from the activeregion and improve the beam property. In order to absorb the straylight, In the ratio x₃ is set to 0.01 or higher, preferably 0.05 orhigher, and more preferably 0.1 or higher. However, it is preferable tocontrol the In ratio to 0.5 or lower, preferably 0.3 or lower, morepreferably 0.2 or lower and most preferably 0.15 or lower.

The current blocking layer 30 is preferably grown at a low temperature,600° C. or lower, for example, so that it is grown with lowcrystallinity. Growing the current blocking layer 30 at a lowtemperature makes it easier to process it by etching with an alkalinesolution or the like and improves the current blocking effect. When thecurrent blocking layer 30 is too thin, it cannot perform the currentblocking function sufficiently, while the light confinement effectbecomes weaker and the threshold increases. When the current blockinglayer 30 is too thick, on the other hand, the influence of the stepbecomes greater, it becomes difficult to form a flat surface whenre-growing and it also becomes difficult to form the cladding layer insuper lattice (SL) structure. Thus the thickness of the current blockinglayer 30 is preferably from 100 to 500 Å, and more preferably from 150to 300 Å.

It is also preferable to form the current blocking layer 30 so that theend face thereof in the longitudinal direction 30 a is located inwardfrom the end face 2 a of the resonator of the laser device 2, as shownin FIG. 5. When the current blocking layer 30 is formed short of the endface 2 a of the resonator, energy density at the end face 2 a of theresonator becomes lower and COD (catastrophic optical damage)characteristic can be improved. It provides also such a benefit thatdefective shape and cracks become less likely to occur in the waveguidewhen forming the end face of resonator by RIE or cleavage. In case theresonator surface is formed by etching, it becomes easier to form a flatresonator surface when the current blocking layer 30 is formed apartfrom the end face of the resonator. This is because, since there is thestep in the window of the current blocking layer 30, it becomesdifficult to form flat etching surface due to the influence of the stepwhen the current blocking layer 30 reaches the end face of theresonator.

Side face of the current blocking layer 30 in the lateral direction isalso preferably formed inward of the side face of the laminate thatconstitutes the stripe structure of the laser device 2 as shown in FIG.5. The current blocking layer having high a mixed crystal ratio of Al isdifficult to etch uniformly, and is likely to have rough surface formedby etching. As a result, in case the current blocking layer has beenformed with the same surface area as the other nitride semiconductorlayers, etching the laminate of the nitride semiconductor layers to forman n electrode may cause rough etching surface and higher connectionresistance of the n electrode. If the current blocking layer 30 isformed within a region inward from the side face of the stripestructure, it becomes easier to carry out etching to form the nelectrode and the resistance thereof can be decreased.

In the meantime, the current blocking layer has a high a mixed crystalratio of Al and therefore has lattice constant and thermal expansioncoefficient that are significantly different from those of the layerslocated above and below thereof. When the current blocking layer 30 isformed within a region located apart from the end face and/or the sideface of the laminate that constitutes the stripe structure to such anextent that does not affect the current blocking effect and the lightconfinement function, namely within the inner region, strain can bereduced and cracks can be suppressed from occurring.

[Re-Growth Layer]

If the semiconductor layers that are re-grown while filling the window32 of the current blocking layer 30 (hereafter called the re-growthlayers) are made of a nitride semiconductor which essentially does notcontain Al, preferably GaN, it becomes easier to fill in the widow 32into a flat surface and the problem of uneven distribution of Al in there-growth layer formed in the window can be solved. Uneven distributionof Al has greater influence on the laser device characteristic than theuneven concentration of impurities such as Mg. The layer re-grown byfilling the window 32 is preferably an optical guide layer rather than acladding layer. By forming the re-growth layer as the optical guidelayer, it becomes easier to fill in the window and grow flat, and it ismade possible to improve the characteristics of the cladding layer ofsuper lattice structure.

Especially when forming the current blocking layer 30 on the p side ofthe active layer, there are several preferred conditions for thesemiconductor layer that is re-grown while filling the window 32 of thecurrent blocking layer 30. First, the re-growth layer preferably has arefractive index comparable to or lower than that of the layer thatmakes contact with the bottom of the first semiconductor layer 22. Thisfurther improves the effect of confining light in the active layer. Itis also preferable to grow the re-growth layer at such a temperaturethat is comparable to or higher than that of the layer that makescontact with the bottom of the first semiconductor layer 22 and issuitable for maintaining the crystallinity of the active layer. Highergrowing temperature improves the crystallinity of the re-growth layerand decreases the resistance. In addition, the re-growth layerpreferably contains impurities in a concentration that is comparable toor higher than that of the layer that makes contact with the bottom ofthe first semiconductor layer 22. Higher impurity concentration in there-growth layer stabilizes the higher mode of laser oscillation as thestray light is absorbed in portions on both sides of the window. There-growth layer may also be advantageously turned into p-type so as todecrease the operating voltage, by intentionally adding a p-typeimpurity such as Mg to the re-growth layer.

[Active Layer 10]

The active layer 10 is preferably made of gallium nitride semiconductorwhere at least the light emitting region contains In, and morepreferably has such a multiple quantum well structure (MQW structure) asIn_(x1)Ga_(1-x1)N well layers (0<X₁<1) and In_(x2)Ga_(1-x2)N barrierlayers (0≦X₂<1, X₁>X₂) are stacked alternately a required number oftimes. The well layer is preferably formed undoped, while all barrierlayers are doped with n-type impurity such as Si or Sn in concentrationfrom 1×10¹⁷ to 1×10¹⁹/cm³. Doping the barrier layers with the n-typeimpurity increases the initial electron concentration in the activelayer, thereby improving the efficiency of injecting electrons into thewell layer, thereby improving the efficiency of the laser to emit light.The active layer 10 may end with a well layer or end with a barrierlayer. Since the active layer 10 contains a relatively high content ofInN that has a high vapor pressure to form a mixed crystal, it is easyto decompose and can be grown at a lower temperature (not higher thanabout 900° C.) than the other layers.

[Carrier Confinement Layer 11]

The carrier confinement layer 11 is made of p-type gallium nitridesemiconductor having a higher mixed crystal ratio of Al than the p-sidecladding layer 14, and preferably has a composition of Al_(c)Ga_(1-c)N(0.1≦c≦0.5). Thickness of the carrier confinement layer 11 is preferablyfrom 50 to 200 Å. The carrier confinement layer 11 is doped with p-typeimpurity such as Mg in a high concentration, preferably from 5×10¹⁷ to1×10¹⁹/cm³. With such a constitution, the carrier confinement layer 11can effectively confine electrons within the active layer and decreasesthe threshold of the laser. The carrier confinement layer 11 also hasthe function to protect the active layer 10 that contains In andtherefore easily decomposes. That is, since the carrier confinementlayer 11 is made of AlGaN that has high decomposition temperature, itcan effectively protect the active layer 10 from decomposition. Thecarrier confinement layer 11 is preferably grown at a low temperaturenot higher than 900° C. in an inert gas such as nitrogen so that theactive layer 10 will not be decomposed.

[N-Side Optical Guide Layer 8, P-Side Optical Guide Layer 12]

The n-side optical guide layer 8 and the p-side optical guide layer 12are preferably made of gallium nitride semiconductor that essentiallydoes not contain Al. Preferably, these layers are made ofIn_(d)Ga_(1-d)N (0.1≦d≦1), and more preferably made of GaN. Whenembedding the current blocking layer in the p-side optical guide layer,the p-side optical guide layer is divided into two layers of firstp-side optical guide layer 12 a and second p-side optical guide layer 12b, and the current blocking layer 30 is formed between these layers. Byforming the second p-side optical guide layer 12 b in such a compositionthat does not essentially contain Al, it becomes easier to achieve aflat surface when the current blocking layer 30 is embedded. Unevendistribution of Al in the window can also be eliminated. The firstp-side optical guide layer 12 a and the second p-side optical guidelayer 12 b may have different compositions and may be formed indifferent processes. The second p-side optical guide layer 12 b, inparticular, preferably has lower refractive index and higher impurityconcentration and is grown at a higher temperature than the first p-sideoptical guide layer. The same applies also to a case of embedding thecurrent blocking layer 30 in the n-side optical guide layer.

[N-Side Cladding Layer 6, P-Side Cladding Layer 14]

The n-side cladding layer 6 and the p-side cladding layer 14 arepreferably formed by stacking nitride semiconductor layers, that havedifferent levels of band gap energy of which at least one includesnitride semiconductor layer that contains Al, in super latticestructure. For the nitride semiconductor layer that contains Al,Al_(e)Ga_(1-e)N (0<e<1) is preferably used. More preferably, superlattice structure is formed by stacking GaN and AlGaN. Since the mixedcrystal ratio of Al of the entire cladding layer can be increased byforming the n-side cladding layer 6 and the p-side cladding layer 14 insuper lattice structure, threshold of the laser can be decreased.Moreover, number of pits formed in the cladding layer can be decreasedby forming it in super lattice structure. Crystallinity can also beimproved by modified doping, wherein one of the layers that constitutethe super lattice structure is doped more heavily than another. Dopingmay also be applied similarly to both layers.

[P-Side Ohmic Electrode]

A p-side ohmic electrode 20 is formed on the p-side contact layer 16. Asthe material to form the p-side electrode 20, Ni, Co, Fe, Cr, Al, Cu,Au, W, Mo, Ta, Ag, Pt, Pd, Rh, Ir, Ru, Os and oxides and nitrides ofthese elements may be used, in the form of single layer, alloy ormulti-layer film. Preferably, at least one kind selected from among Ni,Co, Fe, Cu, Au and Al, or oxide or nitride of these elements may beused. Such a 2-layer structure is preferable that comprises an ohmicelectrode provided in contact with the semiconductor layer and a padelectrode provided thereon. When multiple-layer film is employed,desirable combination is Ni/Au/Pt, Ni/Au/Rh oxide, Pd/Pt, Ni/Au, Co/Auor the like. It is also preferable that such a structure as describedabove is formed as the ohmic electrode that makes contact with thesemiconductor layer, and the pad electrode is formed separately thereon.The pad electrode may also be made of a material similar to thosedescribed above, and it is preferable to use a metal of platinum groupor oxide thereof in the interface with the ohmic electrode whichimproves thermal stability.

Width of the p-side ohmic electrode 20 is preferably larger than thewidth of the window 32 and smaller than the width of the currentblocking layer 30 (total width including window). By forming the p-sideohmic electrode 20 with such a width, it is made possible to efficientlyinject current into the window. Length of the p-side ohmic electrode 20in the direction substantially parallel to the laser beam guidingdirection is preferably shorter than the length of the current blockinglayer 30. By forming the p-side ohmic electrode 20 with such a length,it is made possible to more efficiently inject current into the window.

[N-Side Electrode 18]

As the material to form the n-side electrode 18, Ni, Co, Fe, Ti, Cu, Au,W, Zr, Mo, Ta, Al, Ag, Pt, Pd, Rh, Ir, Ru, Os or the like may be used,in the form of single layer, alloy or multi-layer film. Preferably,Ti/Al, V/Al, V/Pt/Au, Ti/Mo/Ti/Pt/Au or Ti/W/Ti/Pt/Au may be used.Ti/Mo/Ti/Pt/Au and Ti/W/Ti/Pt/Au are especially preferable in case heavyload is applied to the electrode during high output operation since theyare laminates wherein a layer made of a material having high meltingpoint such as Pt is interposed, thus such constitutions are thermallystable.

Now a method of manufacturing the gallium nitride semiconductor laseraccording to this embodiment will be described.

FIG. 4 is a process diagram showing the method of manufacturing thegallium nitride semiconductor laser according to this embodiment. First,as shown in FIG. 4A, after forming a semiconductor layer thatconstitutes the gallium nitride semiconductor laser device to about ahalf of the total thickness of the p-side optical guide layer 12 (namelythe first p-side optical guide layer 12 a) on a wafer in a reactionfurnace of a vapor phase growth apparatus such as MOCVD apparatus, thefirst semiconductor layer 22 made of In_(x1)Al_(y1)Ga_(1-x1-y1)N(0≦x₁≦0.1, y₂<y₁≦y₃, 0<x₁+y₁<1), the second semiconductor layer 24 madeof In_(x2)Al_(y2)Ga_(1-x2-y2)N (0≦x₂≦1, 0≦y₂<y₁, 0≦x₂+y₂<1), and thecurrent blocking layer 30 made of In_(x3)Al_(y3)Ga_(1-x3-y3)N (0≦x₃≦0.1,0.5≦y₃≦1, 0.5≦x₃+y₃≦1) are grown successively.

The first semiconductor layer is preferably grown at a high temperaturecomparable to that for growing the n-side or p-side cladding layer ofthe laser, for example 1000° C. or higher. By growing the firstsemiconductor layer at a high temperature, it is made possible toimprove the crystallinity so as to achieve higher durability againstetch-back and decrease the resistance so as to improve the efficiency ofcurrent injection into the active layer. The second semiconductor layer24 and the current blocking layer 30, on the other hand, are preferablygrown at a low temperature below 1000° C., preferably 600° C. or lower.

Then as shown in FIG. 4B, the wafer is taken out of the reaction vesselof the vapor phase growth apparatus and the window 32 is formed in thecurrent blocking layer 30 by photolithography using a photo resist 34.Etching of the current blocking layer 30 is preferably carried out bywet etching that is less likely to cause damage to the device, ratherthan dry etching. The gallium nitride semiconductor having high Al ratiosuch as AlN, for example, easily dissolves in an alkaline developersolution such as tetramethylammonium hydroxide (TMAH), and therefore canbe processed to pattern the current blocking layer 30 byphotolithography using an alkaline solution as the developer. The secondsemiconductor layer 24, on the other hand, has low Al ratio andtherefore does not dissolve in an alkaline solution, and functions as anetching stopper layer. At the same time, the second semiconductor layer24 also protects a portion of the semiconductor layer located in thewaveguide from the alkaline solution and the oxygen contained in theatmosphere.

Then as shown in FIG. 4C, after removing the photo resist 34, the waferis put into the reaction furnace of the vapor phase growth apparatusagain, and is kept at a high temperature of 1000° C. or higher whileflowing a reducing gas such as hydrogen, so as to carry out etch-back.While the second semiconductor layer 24 has been damaged by oxygencontained in the atmosphere during the process shown in FIG. 4B, it ismade of gallium nitride semiconductor having high Al ratio and istherefore easily removed from the portion exposed through the window 32by etch-back. The first semiconductor layer 22, on the other hand, ismade of gallium nitride semiconductor having high Al ratio and thereforedoes not easily decompose even in a reducing gas such as hydrogen at ahigh temperature, and functions as the etching stopper layer againstetch-back.

Then as shown in FIG. 4D, the second p-side optical guide layer 12 b isgrown on the current blocking layer 30 so as to fill the window 32 andmake a flat surface. At this time, the window 32 can be easily filled tobe flat when the second p-side optical guide layer 12 b is made of anitride semiconductor that essentially does not contain Al, preferablyGaN. The second p-side optical guide layer 12 b that is thesemiconductor used to fill the window 32 of the current blocking layer30 preferably has refractive index comparable to or lower than that ofthe first p-side optical guide layer 12 a that makes contact with thebottom of the first semiconductor layer. With this constitution, moreeffective confinement of light can be achieved. Moreover, the secondp-side optical guide layer 12 b that is the semiconductor used to fillthe window 32 of the current blocking layer 30 is preferably grown atsuch a temperature that is comparable to or higher than that of growingthe first p-side optical guide layer 12 a that is the layer which makescontact with the bottom of the first semiconductor layer and is suitablefor maintaining the crystallinity of the active layer. Higher growingtemperature improves the crystallinity of the semiconductor layer whichis re-grown while filling in the window, and decreases the resistance.After the second p-side optical guide layer 12 b, the p-side claddinglayer 14 and the p-side contact layer 16 may be grown successively bythe ordinary method of manufacturing the gallium nitride semiconductorlaser.

Embodiment 2

While the current blocking layer 30 is formed in the p-side opticalguide layer 12 in the first embodiment, the current blocking layer 30 isformed in the n-side optical guide layer 8 in this embodiment. Withother respects, this embodiment is similar to the first embodiment.

FIG. 6 is a sectional view showing a gallium nitride semiconductor laseraccording to the second embodiment. In this embodiment, the currentblocking layer 30 having the window 32 of stripe configuration is formedin the n-side optical guide layer 8. The current blocking layer 30 ismade of In_(x3)Al_(y3)Ga_(1-x3-y3)N (0≦x₃≦0.1, 0.5≦y₃≦1, 0.5≦x₃+y₃≦1)having high resistivity, and plays the role of controlling thehorizontal transverse mode of the laser oscillation by concentratingcurrent in the window 32.

The n-side optical guide layer 8 comprises a first n-side optical guidelayer 8 a that is located below the current blocking layer 30 and asecond n-side optical guide layer 8 b. The current blocking layer 30 isformed above the first n-side optical guide layer 8 a via the firstsemiconductor layer 22 and the second semiconductor layer 24, while thewindow 32 is formed to penetrate the current blocking layer 30 and thesecond semiconductor layer 24. The second n-side optical guide layer 8 bis formed so as to fill the window 32.

Similarly to the first embodiment, the second semiconductor layer 24 ismade of gallium nitride semiconductor having lower Al ratio than thoseof the current blocking layer 30 and the first semiconductor layer, soas to function as an etching stopper layer when the window 32 is formedin the current blocking layer 30 by photolithography and protects thefirst semiconductor layer 22 from the ambient gas such as oxygen, to beeventually removed in the etch-back process carried out in the vaporphase growth apparatus. The first semiconductor layer 22 functions asthe etching stopper layer during the etch-back process carried outbefore re-growing onto the current blocking layer 30 in the vapor phasegrowth apparatus, and protects the first n-side optical guide layer 8 alocated below thereof from gas etching.

Thus in the gallium nitride semiconductor laser according to thisembodiment, too, since the first semiconductor layer and the secondsemiconductor layer act in a complementary manner, the layer formedthrough reaction with the atmosphere is prevented from remaining in thewindow of the current blocking layer 30 and defective shape can beprevented from being formed due to excessive etch-back, so that stablelaser characteristic can be obtained.

Embodiment 3

In the first embodiment, the second semiconductor layer having lower Alratio is formed below the current blocking layer, and the firstsemiconductor layer having higher Al ratio is formed further belowthereof, so as to prevent excessive etch-back from occurring by means ofthe first semiconductor layer. In this embodiment, only a growth baselayer having low Al ratio is provided below the current blocking layer,and crystallinity of the growth base layer is controlled to be lowerthan that of the layer that makes contact with the bottom of the growthbase layer, so as to prevent excessive etch-back. With other respects,this embodiment is similar to the first embodiment.

FIG. 7 is a sectional view showing the gallium nitride semiconductorlaser according to the third embodiment, the n-side contact layer 4 madeof GaN, the n-side cladding layer 6 made of AlGaN, the n-side opticalguide layer 8 made of GaN, the multiple quantum well active layer 10having a well layer containing In, the p-type optical guide layer 12made of GaN, the p-side cladding layer 14 made of AlGaN and the p-sidecontact layer 16 made of GaN are formed on the substrate 2 made ofsapphire or the like. Formed in the p-type optical guide layer 12 is thecurrent blocking layer 30 that has the stripe-shaped window 32. Thecurrent blocking layer 30 is made of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1,0.5≦y≦1, 0.5≦x+y≦1) that has high resistivity, so as to concentrate thecurrent in the active layer 10 within the window 32 and controlhorizontal transverse mode of laser oscillation.

The p-side optical guide layer 12 comprises the first p-side opticalguide layer 12 a located below the current blocking layer 30 and thesecond p-side optical guide layer 12 b. The current blocking layer 30 isformed on the first p-side optical guide layer 12 a via the growth baselayer 26, and the window 32 is formed to penetrate the current blockinglayer 30 and the growth base layer 26. The second p-side optical guidelayer 12 b is formed so as to fill in the window 32.

The growth base layer 26 is made of a nitride gallium semiconductorhaving Al ratio lower than that of the current blocking layer 30, so asto function as an etching stopper layer when the window 32 is formed inthe current blocking layer 30 by photolithography and also protect thep-type optical guide layer 12 a located below thereof from ambient gassuch as oxygen, before eventually being removed by etch-back carried ina vapor phase growth apparatus.

This is because the growth base layer 26 has low Al ratio (preferably0.05 or less), and it is etched with an alkaline solution at a ratedifferent from that of the current blocking layer 30 that has Al ratioof 0.5 or higher, and therefore remains without being etched when thecurrent blocking layer 30 is etched with alkaline solution. Thus whenthe window 32 is formed in the current blocking layer 30, the growthbase layer 26 serves as the etching stopper layer and prevents excessiveetching. Also because the growth base layer 26 is made of the nitridegallium semiconductor having low Al ratio, it reacts with oxygen andother elements contained in air only at a slow rate. Therefore, thegrowth base layer 26 can effectively protect the first p-side opticalguide layer 12 a located thereunder from the ambient gas such as oxygenin the photolithography process carried out outside the vapor phasegrowth apparatus.

In the meantime, the growth base layer 26 is damaged on the surfacethereof through the etching process by the photolithography and throughexposure to the atmosphere. However, because the growth base layer 26 ismade of a nitride semiconductor that essentially does not contain Al, iteasily decomposes when exposed to a reducing gas such as hydrogen at ahigh temperature. As a result, the growth base layer 26 that has beendamaged can be easily removed from the window 32 that becomes thecurrent path, by etching back in the vapor phase growth apparatus.

However, if there is not a difference in the etching rate between thegrowth base layer 26 and the first p-side optical guide layer 12 a,etch-back tends to proceed excessively in the first p-side optical guidelayer 12 a. In this embodiment, therefore, the growth base layer 26 ismade with lower crystallinity than the first p-side optical guide layerthat makes contact with the bottom thereof, thereby generating adifference in the etching rate for etch-back between these layers. Thatis, by making the growth base layer 26 with lower crystallinity than thefirst p-side optical guide layer, etch-back is caused to proceed fast inthe growth base layer 26 and slowly in the first p-side optical guidelayer 12 a. This makes it possible to selectively remove the growth baselayer 26 only.

Thus in the gallium nitride semiconductor laser according to thisembodiment, too, the layer formed through reaction with the atmosphereis prevented from remaining in the window of the current blocking layer30 and defective shape can be prevented from being formed due toexcessive etch-back, so that stable laser characteristic can beobtained.

The growth base layer 26 is made of gallium nitride semiconductor havinglower Al ratio that the current blocking layer 30, namely a galliumnitride semiconductor having a composition represented by the generalformula In_(x′)Al_(y′)Ga_(1-x′-y′)N (0≦x′≦1, 0≦y′<y, 0≦x′+y′<1). Thelower the Al ratio in the growth base layer 26, the greater thedifference in etching rate in the wet etching process from the currentblocking layer 30, thus making it easier to remove by etch-back. It ispreferable to control the Al ratio y₄ in the growth base layer 26 to 0.1or lower, preferably to 0.05 or lower, and more preferably toessentially zero.

The growth base layer 26 preferably contains In. When the growth baselayer 26 contains In, selective etch-back becomes easy and the effect ofabsorbing stray light leaking from the waveguide can also be achieved.In ratio x′ in the growth base layer 26 is preferably from 0 to 0.2, andmore preferably 0.05 to 0.15. Thus preferable composition of the growthbase layer 26 is In_(f)Ga_(1-f)N (0≦f≦0.2).

In order to make the crystallinity of the growth base layer 26 lowerthan that of the layer that makes contact therewith from below, forexample, In ratio in the growth base layer may be made higher than thatof the layer that makes contact therewith from below, or the growth baselayer may be grown at a lower temperature than the layer that makescontact therewith from below. The higher the In ratio of the growth baselayer 26, the faster it decomposes when exposed to reducing atmospheresuch as hydrogen at a high temperature. As a result, the growth baselayer 26 can be selectively removed when the In ratio of the growth baselayer 26 is higher than that of the first p-side optical guide layer 12a. The lower the growing temperature of the growth base layer 26, on theother hand, the faster it decomposes when exposed to reducing atmospheresuch as hydrogen at a high temperature. As a result, the growth baselayer 26 can be selectively removed even when the growth base layer 26and the first p-side optical guide layer 12 a are made of GaN of thesame composition. Crystallinity of the growth base layer 26 can be madelow also by making the impurity concentration thereof higher than thelayer that makes contact therewith from below.

Capability of the growth base layer 26 to absorb stray light from theactive layer can be increased by forming the growth base layer 26 withhigher impurity concentration (for example, 5×10¹⁸/cm³ or higher) orcontaining In. This makes it possible to suppress the oscillation inhigher mode and generate stable laser beam of single mode.

When selective etch-back is carried out by lowering the growingtemperature of the growth base layer 26, the growing temperature ispreferably set to 900° C. or lower, and more preferably 600° C. orlower. As the means for lowering the crystallinity of the growth baselayer 26, both means of increasing the In ratio and lowering the growingtemperature may be used in combination. The growth base layer 26 ispreferably made in amorphous or polycrystalline structure. By making thegrowth base layer 26 in amorphous or polycrystalline structure, itbecomes easier to remove than single crystal. Also because resistivityof the growth base layer becomes higher, better current blocking effectcan be achieved through combined effect of the high resistivity and thecurrent blocking layer formed in contact thereon. It is also madepossible to make the current blocking layer thinner.

Forming the growth base layer 26 with low crystallinity also has such aneffect that it becomes easier to remove the current blocking layer 30that is grown thereon. When the layer that makes contact with the bottomof the current blocking layer 30 is good, a part of the current blockinglayer 30 near the interface has good crystallinity and becomes difficultto remove by etching. Accordingly, the current blocking layer 30 has lowcrystallinity from the early stage of growth and it becomes easier toremove the current blocking layer 30 from within the window 32, byforming the growth base layer 26 that makes contact with the bottom ofthe current blocking layer 30 with low crystallinity.

The growth base layer 26 cannot protect the first p-type optical guidelayer 12 a sufficiently when it is too thin, and allows the step tocause greater influence when it is too thick. When the step is larger,it becomes difficult to form the semiconductor layer to be re-grown onthe current blocking layer with flat surface. Therefore, the thicknessof the growth base layer 26 is preferably from 10 to 300 Å, and morepreferably from 50 to 200 Å.

The layer that makes contact with the bottom of the growth base layer 26is preferably an optical guide layer. It becomes easier to control theconfinement of light by forming the current blocking structure so as tomake contact with the optical guide layer.

Now a method of manufacturing the gallium nitride semiconductor laseraccording to this embodiment will be described.

FIG. 8 is a process diagram showing the method of manufacturing thegallium nitride semiconductor laser according to the third embodiment.First, as shown in FIG. 8A, after forming a semiconductor layer thatconstitutes the gallium nitride semiconductor laser device to about ahalf of the total thickness of the p-side optical guide layer 12 (namelythe first p-side optical guide layer 12 a) on a wafer in the reactionfurnace of the vapor phase growth apparatus such as MOCVD apparatus, thegrowth base layer 26 made of In_(x)Al_(y)Ga_(1-x′-y′)N (0≦x′≦1, 0≦y′<y,0≦x′+y′<1) and the current blocking layer 30 made ofIn_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1) are grownsuccessively. The growth base layer 26 is formed with lowercrystallinity than that of the first p-side optical guide layer 12 a byeither increasing the In ratio or lowering the growing temperature.

Then as shown in FIG. 8B, the wafer is taken out of the reaction furnaceof the vapor phase growth apparatus and the window 32 is formed in thecurrent blocking layer 30 by photolithography using the photo resist 34.Patterning of the current blocking layer 30 is preferably carried out byphotolithography using an alkaline developer solution such astetramethylammonium hydroxide (TMAH). The growth base layer 26 has lowAl ratio and is therefore difficult to dissolve in tetramethylammoniumhydroxide (TMAH), so as to function as an etching stopper layer. At thesame time, the growth base layer 26 also protects the portion of thesemiconductor layer located in the waveguide from oxygen contained inthe atmosphere.

Then as shown in FIG. 8C, after removing the photo resist 34, the waferis put into the reaction furnace of the vapor phase growth apparatusagain and is kept at a high temperature of 1000° C. or higher whileflowing a reducing gas such as hydrogen, so as to carry out etch-back.While the growth base layer 26 has been damaged by oxygen or otherelement contained in the atmosphere during the process shown in FIG. 8B,it is formed so as to have lower crystallinity than the first p-sideoptical guide layer 12 a and therefore the portion thereof exposedthrough the window 32 is selectively removed by etch-back. At this time,the first p-side optical guide layer 12 a functions as the etchingstopper layer against etch-back.

Then as shown in FIG. 8D, the second p-side optical guide layer 12 b isgrown on the current blocking layer 30 so as to fill the window 32 tomake a flat surface. At this time, the window 32 can be easily filled tobe flat when the second p-side optical guide layer 12 b is made of anitride semiconductor that essentially does not contain Al, preferablyGaN. The second p-side optical guide layer 12 b that is thesemiconductor used to fill the window of the current blocking layer 30preferably has refractive index comparable to or lower than that of thefirst p-side optical guide layer 12 a that makes contact with the bottomof the growth base layer. With this constitution, more effectiveconfinement of light can be achieved. Moreover, the second p-sideoptical guide layer 12 b that is the semiconductor used to fill thewindow of the current blocking layer 30 is preferably grown at such atemperature that is comparable to or higher than that of the layer thatmakes contact with the bottom of the growth base layer and is suitablefor maintaining the crystallinity of the active layer. Higher growingtemperature improves the crystallinity of the semiconductor layer whichis re-grown while filling in the window, and decreases the resistance.After the second p-side optical guide layer 12 b, the p-side claddinglayer 14 and the p-side contact layer 16 may be grown successively bythe ordinary method of manufacturing the gallium nitride semiconductorlaser.

Embodiment 4

While the current blocking layer 30 is formed in the p-side opticalguide layer 12 in the third embodiment, the current blocking layer 30 isformed in the n-side optical guide layer 8 in this embodiment. Withother respects, this embodiment is similar to the third embodiment.

FIG. 9 is a sectional view showing a gallium nitride semiconductor laseraccording to the fourth embodiment. In this embodiment, the currentblocking layer 30 having the window 32 of stripe configuration is formedin the n-side optical guide layer 8. The current blocking layer 30 ismade of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1) havinghigh resistivity, and plays the role of controlling the horizontaltransverse mode of the laser oscillation by concentrating current in thewindow 32.

The n-side optical guide layer 8 comprises the first n-side opticalguide layer 8 a that is located below the current blocking layer 30 andthe second n-side optical guide layer 8 b. The current blocking layer 30is formed above the first n-side optical guide layer 8 a via the growthbase layer 26, and the window 32 is formed to penetrate the currentblocking layer 30 and the growth base layer 26. The second n-sideoptical guide layer 8 b is formed so as to fill the window 32.

Similarly to the third embodiment, the growth base layer 26 is formedfrom nitride semiconductor that essentially does not contain Al, so asto have lower crystallinity than the first n-side optical guide layer 8a. Thus the growth base layer 26 functions as an etching stopper layerwhen the window 32 is formed in the current blocking layer 30 byphotolithography and protects the first n-side optical guide layer 8 afrom the ambient gas such as oxygen, to be eventually removedselectively in the etch-back process carried out in the vapor phasegrowth apparatus.

Thus in the gallium nitride semiconductor laser according to thisembodiment, too, the layer formed through reaction with the atmosphereis prevented from remaining in the window of the current blocking layer30 and defective shape can be prevented from being formed due toexcessive etch-back, so that stable laser characteristic can beobtained.

Embodiment 5

FIG. 10 is a sectional view showing the gallium nitride semiconductorlaser according to this embodiment, The n-side contact layer 4 made ofGaN, the n-side cladding layer 6 made of AlGaN, the n-side optical guidelayer 8 made of GaN, the multiple quantum well active layer 10 having awell layer containing In, the p-type optical guide layer 12 made of GaN,the p-side cladding layer 14 made of AlGaN and the p-side contact layer16 made of GaN are formed on the substrate 2 made of a differentmaterial such as sapphire. Formed in the p-side optical guide layer 12is the current blocking layer 30 that has the stripe-shaped window 32.The current blocking layer 30 is made of gallium nitride semiconductorthat has Al ratio of 0.5 or higher and high resistivity, so as toconcentrate the current in the active layer 10 within the window 32 andcontrol horizontal transverse mode of laser oscillation.

FIGS. 11 A and 11B are partially enlarged sectional views showing thestructure of a region in the vicinity of the current blocking layer 30in more detail. As shown in FIG. 11A, the carrier confinement layer 11made of gallium nitride semiconductor containing Al is formed in a thinfilm of 50 to 200 Å on the active layer 10 made of gallium nitridesemiconductor, and the p-type optical guide layer 2 made of GaN isformed thereon. The p-type optical guide layer 12 comprises the firstp-type optical guide layer 12 a located below the current blocking layer30 and the second p-type optical guide layer 12 b. The current blockinglayer is formed on the first p-type optical guide layer 12 a and thewindow 32 is formed in the current blocking layer 30. The second p-typeoptical guide layer 12 b is formed so as to fill in the window 32.

The current blocking layer 30 blocks the current in a portion 30 a thatencloses the window (hereafter called the main portion), and causes thecurrent to flow through the window 32 into the active layer. The mainportion 30 a of the current blocking layer is not only high inresistance but is also extremely low in crystallinity on the surfacebecause of the Al ratio as high as 0.5. As a result, as shownschematically in FIG. 11A, dislocations 40 occur with a high densityalso in the p-side cladding layer 14 and in the p-side contact layer 16formed above the main portion 30 a of the current blocking layer, thusmaking it difficult for current to flow therein. That is, the currentblocking layer 30 has current blocking effect in the semiconductor layerlocated above the main portion 30 a due to low crystallinity thereof, inaddition to the current blocking effect due to the resistance of themain portion 30 a. Therefore, even when the current blocking layer 30 isformed with a relatively small thickness of several hundreds ofangstrom, it has sufficient current blocking effect through the combinedeffect of high resistance and low crystallinity.

The window 32 of the current blocking layer 30 is formed so that aportion of the current blocking layer 30 that contacts the first p-sideoptical guide layer 12 a, which is the base layer, remains with athickness of 10 Å or more. The portion of the current blocking layerremaining in the window 32 (hereafter referred to simply as theremaining film portion) 30 b has good crystallinity due to the influenceof good crystallinity of the first p-side optical guide layer 12 a, thatis the base layer, and has therefore low resistance. As the result ofgood crystallinity, substantially no dislocations occur above theremaining film portion 30 b of the current blocking layer, as shown inFIG. 11A. Thus current can be injected efficiently into the active layerwithin the window 32, despite the presence of the remaining film portion30 b of the current blocking layer that has high Al ratio.

In the meantime, since the remaining film portion 30 b of the currentblocking layer has high Al ratio and good crystallinity, it decomposesvery slowly in the reducing atmosphere such as hydrogen, and functionsas the etching stopper layer when the oxide layer or the like is removedfrom the wafer surface by etch-back prior to re-growth. Although theremaining film portion 30 b of the current blocking layer 30 is alsodecomposed by the etch-back using hydrogen or the like, the rate ofdecomposition is very slow and therefore even the etch-back process thatis carried out for a sufficient period of time to completely remove theoxide layer or the like formed on the surface of the current blockinglayer 30 does not penetrate the remaining film portion 30 b of thecurrent blocking layer 30 and erode the p-side guide layer 12 a locatedbelow.

Since sufficient etching stopper function cannot be obtained when theremaining film portion 30 b of the current blocking layer is too thin, amean thickness remaining after etch-back is set to 10 Å or more,preferably 30 Å or more. When the remaining film portion 30 b of thecurrent blocking layer is too thick, crystallinity of the remaining filmportion 30 b becomes lower and, as a result, resistance increases and/ordislocations occur, resulting in higher threshold current of laseroscillation. Therefore, a mean thickness remaining after etch-back ofthe remaining film portion 30 b is preferably less than 100 Å, and morepreferably less than 70 Å.

Since excessive etching is prevented by the remaining film portion 30 bof the current blocking layer, flatness of the layer formed on thecurrent blocking layer 30 and the device characteristics are improved asshown in FIG. 11A. Decrease in the step caused by leaving the remainingfilm portion 30 b and low crystallinity of the main portion 30 a of thecurrent blocking layer 30 also contribute to the improvement inflatness, in addition to the etching stopping effect of the remainingfilm portion 30 b. That is, as the step in the window 32 of the currentblocking layer is decreased by leaving the remaining film portion 30 b,it is made easier to eliminate the step by filling with the secondp-type optical guide layer 12 b. Also the crystal grows at a faster ratein a region 36 located above the remaining film portion 30 b than in theregion 38 located above the main portion 30 a, since the main portion 30a of the current blocking layer has lower crystallinity and theremaining film portion 30 b has good crystallinity. As a result, asshown in FIG. 11A, the window 32 which is a recess can be easily filledand made flat with the second p-side optical guide layer 12 b.

As the second p-side optical guide layer 12 b is formed flat,compositions of the p-side cladding layer 14 and the p-side contactlayer 16 formed thereon can be restricted from becoming uneven, so thatthe functions of the layers are improved. In case the p-side claddinglayer 14 has super lattice structure, in particular, the super latticestructure is disturbed when there is a step on the surface of the secondp-side optical guide layer 12 b that is the base layer, and therefore itis important that the second p-side optical guide layer 12 b fills thewindow 32 to make a flat surface.

Moreover, the device layers (the second p-side optical guide layer, thep-side cladding layer and the p-side contact layer) in the region 36above the remaining film portion 30 b may also be formed to be thickerthan the region 38 located above the main portion 30 a, as shown in FIG.11B. Since the main portion 30 a of the current blocking layer has lowcrystallinity, crystal grows faster in the region 36 located above theremaining film portion 30 b than in the region 38 located above the mainportion 30 a. As a result, the device layers in the region 36 locatedabove the remaining film portion 30 b can be made thicker than thesurrounding area if the difference in growing rate becomes greater.Since the device layers in the region 36 located above the remainingfilm portion 30 b constitute the core portion that surrounds the activeregion of the laser, it is advantageous for confinement of light to makethis portion thicker.

Such a film thickness distribution as shown in FIG. 11A or 11B can beobtained by controlling the crystallinity of the main portion 30 a andthe remaining film portion 30 b of the current blocking layer.Crystallinity of these layers can be controlled by means of such factorsas the Al ratio in the current blocking layer 30, growing temperature,total thickness, thickness of the remaining film portion 30 b,crystallinity of the base layer. The better the crystallinity of thebase layer of the current blocking layer and the thinner the remainingfilm portion 30 b, the higher the crystallinity of the remaining filmportion 30 b of the current blocking layer becomes.

The current blocking layer 30 is preferably carried out at a lowtemperature so that the main portion 30 a has low crystallinity in thesurface thereof. The growing temperature is, for example, preferably900° C. or lower and more preferably 600° C. or lower. Growing thecurrent blocking layer 30 at a low temperature makes it easier toprocess it by etching with an alkaline solution or the like and improvesthe current blocking effect. When the main portion 30 a of the currentblocking layer 30 is too thin, it cannot perform the current blockingfunction sufficiently and, when it is too thick, on the other hand,influence of the step becomes greater. Thus a total thickness of thecurrent blocking layer 30 (that is the thickness of the main portion 30a) is preferably from 100 to 800 Å, and more preferably from 150 to 500Å.

The current blocking layer 30 is formed on a base layer that hascrystallinity higher than at least that of the current blocking layer 30itself. Crystallinity better than that of the current blocking layer, ofwhich purpose is insulation, can be obtained as long as the layerconstitutes an ordinary gallium nitride semiconductor laser and isformed in the current path.

It is also preferable to form the current blocking layer 30 except forthe residual film so that the end face thereof in the longitudinaldirection 30 a is located inward from the end face 2 a of the resonatorof the laser device 2, as shown in FIG. 5. When the current blockinglayer 30 is formed short of the end face 2 a of the resonator, energydensity at the end face 2 a of the resonator becomes lower and COD(catastrophic optical damage) characteristic can be improved. There isalso such a benefit that defective shape and cracks are less likely tooccur in the waveguide when forming the end face of resonator by RIE orcleavage. In case the resonator surface is formed by etching, it becomeseasier to form a flat resonator surface when the current blocking layer30 is formed apart from the end face of the resonator. This is because,since certain level of step is generated in the window of the currentblocking layer 30, it becomes difficult to form flat etching surface dueto the influence of the step when the current blocking layer 30 reachesthe end face of the resonator. As for the remaining film portion of thecurrent blocking layer, there is not problem if it is formed to reachthe end face of the resonator 2 a, since it is a thin film where currentflows.

Side face of the current blocking layer 30 in the lateral direction isalso preferably formed inward of the side face of the laminate thatconstitutes the stripe structure of the laser device 2 as shown in FIG.5. The current blocking layer having high Al ratio is difficult to etchuniformly, and is likely to have rough surface formed by etching. As aresult, in case the current blocking layer has been formed with the samearea as the other nitride semiconductor layers, etching the laminate ofthe nitride semiconductor layers to form an n electrode may cause roughetching surface and higher connection resistance of the n electrode. Ifthe current blocking layer 30 is formed within a region inward from theside face of the stripe structure, it becomes easier to carry outetching to form the n electrode and the resistance decreases.

In the meantime, the current blocking layer has high Al ratio andtherefore has lattice constant and thermal expansion coefficient thatare significantly different from those of the layers located above andbelow thereof. When the current blocking layer 30 is formed within aregion apart from the end face and/or the side face of the laminate thatconstitutes the stripe structure to such an extent that does not affectthe current blocking effect and the light confinement function, namelywithin the inner region, strain can be reduced and cracks can besuppressed from occurring.

Now a method of manufacturing the gallium nitride semiconductor laseraccording to this embodiment will be described.

FIG. 12 is a process diagram showing the method of manufacturing thegallium nitride semiconductor laser according to this embodiment. First,as shown in FIG. 12A, after forming a semiconductor layer thatconstitutes the gallium nitride semiconductor laser to about a half ofthe total thickness of the p-side optical guide layer 12 (namely thefirst p-side optical guide layer 12 a) in the reaction furnace of thevapor phase growth apparatus such as MOCVD apparatus, the currentblocking layer 30 made of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1, 0.5≦y≦1,0.5≦x+y≦1) is grown. The current blocking layer 30 is formed preferablygrown at a low temperature lower than 1000° C. and more preferably 600°C. or lower.

Then as shown in FIG. 12B, the wafer is taken out of the reactionfurnace of the vapor phase growth apparatus and the window 32 is formedin the current blocking layer 30 by photolithography using the photoresist 34. Etching of the current blocking layer 30 is preferablycarried out by wet etching that causes less damage, rather than dryetching. Since the gallium nitride semiconductor having high Al ratiosuch as AlN easily dissolves in an alkaline developer solution such astetramethylammonium hydroxide (TMAH), patterning of the current blockinglayer 30 can be done by photolithography using an alkaline solution asthe developer solution.

The window 32 in the current blocking layer 30 is formed in such amanner that a portion of predetermined thickness of the current blockinglayer 30 remains as the remaining film portion 30 b in the window 32.This can be achieved by making use of the change in crystallinity of thecurrent blocking layer in the direction of thickness. Crystallinity ofthe current blocking layer is good in the portion in contact with thefirst p-side optical guide layer 12 a that is the base layer, andbecomes lower with the distance from the base layer. As a result, thecurrent blocking layer is etched by alkaline solution or the like atdifferent rates between the portion near the base layer and the portionlocated above, and only the portion of the current blocking layer nearthe base layer can be left to remain by properly setting the etchingconditions such as concentration and temperature of the alkalinesolution.

Then as shown in FIG. 12C, after removing the photo resist 34, the waferis put into the reaction furnace of the vapor phase growth apparatusagain and is kept at a high temperature of 1000° C. or higher whileflowing a reducing gas such as hydrogen, so as to carry out etch-back.The layer formed through reaction of the surface of the semiconductorlayer (namely the current blocking layer 30) and oxygen and otherelement in the atmosphere is removed by etch-back in the process shownin FIG. 12B.

At this time, the remaining film portion 30 b of the current blockinglayer functions as the etching stopper layer. That is, since theremaining film portion 30 b of the current blocking layer has high Alratio and good crystallinity, it decomposes slowly in reducingatmosphere such as hydrogen. Therefore, even the etch-back process thatis carried out for a sufficient period of time to completely remove theoxide layer or the like formed on the surface of the current blockinglayer 30 does not penetrate the remaining film portion 30 b of thecurrent blocking layer 30 to erode the p-side guide layer 12 a locatedbelow.

Then as shown in FIG. 12D, the second p-side optical guide layer 12 b isgrown on the current blocking layer 30 so as to fill the window 32 tomake a flat surface. At this time, the window 32 can be easily filled tobe flat when the second p-side optical guide layer 12 b is made of anitride semiconductor that essentially does not contain Al, preferablyGaN. After the second p-side optical guide layer 12 b, the p-sidecladding layer 14 and the p-side contact layer 16 may be grownsuccessively by the ordinary method of manufacturing the gallium nitridesemiconductor laser.

Embodiment 6

While the current blocking layer 30 is formed in the p-side opticalguide layer 12 in the fifth embodiment, the current blocking layer 30 isformed in the n-side optical guide layer 8 in this embodiment. Withother respects, this embodiment is similar to the fifth embodiment.

FIG. 13 is a sectional view showing a gallium nitride semiconductorlaser according to the sixth embodiment. In this embodiment, the currentblocking layer 30 having the window 32 of stripe configuration is formedin the n-side optical guide layer 8. The current blocking layer 30 ismade of In_(x)Al_(y)Ga_(1-x-y)N (0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1) havinghigh resistivity, and plays the role of controlling the horizontaltransverse mode of the laser oscillation by concentrating current in thewindow 32.

The n-side optical guide layer 8 comprises the first n-side opticalguide layer 8 a that is located below the current blocking layer 30 andthe second n-side optical guide layer 8 b. The current blocking layer 30is formed above the first n-side optical guide layer 8 a, and the window32 is formed so that the portion of a predetermined thickness of thecurrent blocking layer 30 near the base layer remains. Then the secondn-side optical guide layer 8 b is formed so as to fill the window 32.

Similarly to the fifth embodiment, the remaining film portion 30 b ofthe current blocking layer functions as an etching stopper layer duringthe etch-back process carried out prior to re-growing onto the currentblocking layer 30 in the vapor phase growth apparatus, and alsodecreases the step in the window 32. Since the remaining film portion 30b of the current blocking layer has low resistance and goodcrystallinity, it does not block the current injection into the activelayer 10. Consequently, excellent laser characteristic can be achievedstably with the gallium nitride semiconductor laser of this embodiment.

As described in the first and sixth embodiments, the current blockinglayer 30 may be formed either on the p side or n side of the activelayer according to the present invention. With ordinary gallium nitridesemiconductor, since the p-type gallium nitride semiconductor has highresistivity, n-side layer, active layer and p-side layer are grown inthis order. Therefore, in case the current blocking layer 30 is formedon n side of the active layer as in the second, fourth and sixthembodiments, dislocations originating in the current blocking layer 30of low crystallinity pass through the active layer 10. Whensatisfactorily flat surface is not formed with the second n-side opticalguide layer 8 b, the active layer 10 cannot be formed flat and leakagecurrent is likely to flow. For these reasons, it is preferable to formthe current blocking layer on the p side of the active layer as in thefirst, third and fifth embodiments. When the current blocking layer isformed on the p side of the active layer, current flow into the activelayer can be efficiently controlled and therefore the laser canoscillate efficiently. Also because the wafer is taken out of the gasphase growing furnace after growing the layers continuously up to theactive layer, damage to the active layer is reduced.

In case the current blocking layer is formed on the n side of the activelayer, on the other hand, there occur many dislocations on both sides ofthe active layer above the window. Since the portion of dislocationcontains much In content, it makes over-saturated absorption region dueto higher In ratio than in the light emission region. As a result,pulsation laser can be made.

While the current blocking layer 30 is formed in the optical guide layerin the first through sixth embodiments, the present invention is notlimited to this constitution. For example, the current blocking layermay be formed between the optical guide layer and the cladding layer, orinside of the cladding layer. When the current blocking layer 30 is toodistant from the active layer, however, the current that has beenconcentrated by the current blocking layer 30 tends to spread beforereaching the active layer 10. Therefore, it is preferable that thecurrent blocking layer is located near the active layer 10 to such anextent as crystallinity of the active layer 10 is not adverselyaffected. It is advantageous for the purpose of achieving flat surfacethat the layer formed on the current blocking layer 30 is made ofgallium nitride semiconductor that essentially does not contain Al. Thusit is preferable to form the current blocking layer 30 in the opticalguide layer made of GaN in order to achieve the current blocking effectand flat surface, as described in conjunction with this embodiment.

The nitride semiconductor laser device according to the presentinvention can be packaged by either face-up mounting or face-downmounting.

EXAMPLES

Now examples of the present invention will be described.

Example 1

In Example 1, the gallium nitride semiconductor laser having thestructure shown in FIG. 1 is fabricated.

(Substrate 2)

A substrate made of sapphire with the principal plane lying in the Cplane having diameter of 2 inches is set in a MOVPE reaction vessel, thetemperature is set to 500° C., and a buffer layer made of GaN is formedto a thickness of 200 Å by using trimethyl gallium (TMG) and ammonia(NH₃). Then temperature is raised and an undoped GaN layer is grown witha thickness of 1.5 μm. Then a plurality of stripe-shaped masks areformed on the surface of the GaN layer, and GaN is grown selectivelythrough the windows of the masks, so as to grow GaN layer by crystalgrowth accompanied by lateral growth (ELOG). The mask used in selectivegrowing is made of SiO₂, the mask measuring 15 μm in width and thewindow measuring 5 μm in width.

(Buffer Layer)

Then with the temperature raised to 1050° C., a buffer layer (not shown)made of AlGaN is formed with a thickness of 4 μm on the substrate 2 byusing TMG (trimethyl gallium), TMA (trimethyl ammonium) and ammonia.This layer functions as the buffer layer between the n-type contactlayer 4 which is formed next and the substrate 2.

(N-Side Contact Layer 4)

The N-Type Contact Layer 4 Made of AlGaN doped with Si is grown with athickness of 4 μm at a temperature of 1050° C. on the buffer layer byusing TMG, TMA, ammonia, and silane gas used as an impurity gas.

(Crack Preventing Layer)

Then a crack preventing layer (not shown) made of InGaN is grown with athickness of 0.15 μm at a temperature of 900° C. by using TMG, TMI(trimethyl indium), and ammonia.

(N-Type Cladding Layer 6)

Then with the temperature set to 1050° C., after growing layer A made ofundoped AlGaN with a thickness of 25 Å by using TMA, TMG and ammonia asthe stock material gas, supply of TMA is stopped and silane gas is usedas the impurity gas, to form layer B made of GaN doped with Si inconcentration of 5×10¹⁸/cm³ with a thickness of 25 Å. These operationsare repeated to form the n-type cladding layer 6 made in multi-layeredfilm (super lattice structure) having a total thickness of 1 μm. At thistime, the cladding layer can function satisfactorily when the mix ratioof Al of the undoped AlGaN is in a range from 0.05 to 0.3.

(N-Side Optical Guide Layer 8)

Then at temperature of 1050° C., the n-type optical guide layer 8 madeof undoped GaN is formed with a thickness of 0.15 μm by using TMG andammonia as the stock material gas. This layer may also be doped with ann-type impurity.

(Active Layer 10)

Then with the temperature set to 900° C., a barrier layer (B) made ofIn_(0.05)Ga_(0.95)N doped with Si in a concentration of 5×10¹⁸/cm³ isformed to a thickness of 140 Å by using TMI (trimethyl indium), TMG andammonia as the stock material gas and silane gas as the impurity gas.Then the supply of silane gas is stopped and a well layer (W) made ofundoped In_(0.1)Ga_(0.9)N is grown with a thickness of 40 Å. The barrierlayers (B) and the well layers (W) are stacked one on another in theorder of barrier layer/well layer/barrier layer/well layer/ . . ./barrier layer/well layer/barrier layer. While the last layer may beeither barrier layer or well layer, it is preferable that barrier layeris formed as the last layer. The active layer 10 is made in multiplequantum well structure (MQW) having total thickness of about 500 Å.

(Carrier Confinement Layer)

Then at a similar temperature, the p-side carrier confinement layer 11(not shown in FIG. 1) made of AlGaN doped with Mg in a concentration of1×10¹⁹/cm³ is formed with a thickness of 100 Å by using TMA, TMG andammonia as the stock material gas and Cp₂Mg (cyclopentadienyl magnesium)as the impurity gas. Providing this layer improves the effect ofconfining electrons and protects the active layer 10 from decomposition.

(The First P-Side Optical Guide Layer 12 a)

Then with the temperature set to 1000° C., the first p-side opticalguide layer 12 a made of undoped GaN is grown with a thickness of 0.075μm by using TMG and ammonia as the stock material gas. Although thefirst p-side optical guide layer 12 a is grown undoped, Mg concentrationtherein reaches 5×10¹⁶/cm³ so as to show p-type property due to thediffusion of Mg from adjacent layers such as the p-side carrierconfinement layer 11 and the p-side cladding layer 14. This layer mayalso be grown while intentionally doping it with Mg.

(First Semiconductor Layer 22, Second Semiconductor Layer 24)

Then while maintaining the temperature at 1000° C., the firstsemiconductor layer 22 made of Al_(0.2)Ga_(0.8)N doped with Mg is formedwith a thickness of 70 Å by using TMA, TMG and ammonia as the stockmaterial gas. With the temperature lowered to 800° C., the secondsemiconductor layer made of In_(0.08)Ga_(0.92)N is grown with athickness of 100 Å by using TMI, TMG and ammonia as the stock materialgas.

(Current Blocking Layer 30)

Then with the temperature set to 500° C., the current blocking layer 30made of AlN is formed with a thickness of 300 Å by using TMG and ammoniaas the stock material gas. The wafer having the layers described aboveformed thereon is taken out of the furnace of the reaction furnace ofthe MOCVD apparatus and is processed to form the stripe-shaped window 32as follows. First, the current blocking layer 30 is coated with a photoresist over substantially the entire surface thereof. After the surfaceis exposed to light in the pattern of the window 32, the pattern isdeveloped using TMAH that is an alkaline solution. Since the AlN layer30 dissolves in the alkaline developer solution, the portion of the AlNlayer 30 located above the window 32 is etched and removed during thedeveloping process (FIG. 4B). On the other hand, the secondsemiconductor layer made of In_(0.08)Ga_(0.92)N does not dissolve in thealkaline solution and therefore functions as an etching stopper layeragainst etching of the AlN layer 30. Remaining resist film is removed byashing, and the wafer is washed in acid.

Then the wafer is put into the reaction furnace of the MOCVD apparatusof which temperature is set to 1000° C. and hydrogen gas, which is areducing gas, is blown onto the surface so as to carry out etch-back.Since the current blocking layer 30 made of AlN and the firstsemiconductor layer 22 having Al ratio of 0.2 have high decompositiontemperatures, these layers are decomposed by the etch-back more slowlythan the second semiconductor layer 24 that is made of InGaN. As aresult, the portion of the second semiconductor layer 24 exposed throughthe window 32 is selectively removed by the etch-back process.

(Second P-Side Optical Guide Layer 12B)

Then with the temperature set to 1000° C., the second p-side opticalguide layer 12 b made of undoped GaN is formed to a thickness of 0.075μm by using TMG and ammonia as the stock material gas. The second p-sideoptical guide layer 12 b is grown while doping with Mg, Since the secondp-side optical guide layer 12 b does not contain Al, it easily grows toform a flat surface while filling the window 32 thereby eliminating thestep.

(P-Side Cladding Layer 14)

Then at temperature of 1000° C., a layer made of undoped AlGaN is formedwith a thickness of 25 Å. Then the supply of TMA is stopped, and a layermade of GaN doped with Mg is formed with a thickness of 25 Å by usingCp₂Mg. These operations are repeated 90 times so as to form the p-sidecladding layer 14 of super lattice structure having total thickness of0.45 μm.

(P-Side Contact Layer 16)

Last, at temperature of 1000° C., the p-side contact layer 16 made ofp-type GaN doped with Mg in a concentration of 1×10²⁰/cm³ is formed witha thickness of 150 Å on the p-side cladding layer 14. The p-side contactlayer 16 can be made of p-type gallium nitride semiconductor, and ispreferably made of GaN doped with Mg which enables it to achieve mostpreferable ohmic contact with the p electrode 20. Since the p-sidecontact layer 16 is the layer that forms an electrode, it is desirablyformed to achieve a high carrier concentration of 1×10¹⁷/cm³ or higher.When the carrier concentration is lower than 1×10¹⁷/cm³, it becomesdifficult to attain satisfactory ohmic contact with the electrode. Whenthe reaction is completed, the wafer is annealed at in nitrogenatmosphere in the reaction vessel at temperature of 700° C., so as tofurther decrease the resistivity of the p-type layers.

After forming the layers one on another by growing the nitridesemiconductor as described above, the wafer is taken out of the reactionvessel and a protective layer (not shown) made of SiO₂ is formed on thesurface of the p-type contact layer 16 at the top. Surface of the n-sidecontact layer 4 is exposed in a region where the n electrode is to beformed as shown in FIG. 1 by etching in the RIE (reactive ion etching)process. SiO₂ is best suited as the protective film for deep etching ofthe nitride semiconductor.

Then the stripe-shaped p electrode 20 made of Ni/Au is formed on thesurface of the p-side contact layer 16 and the stripe shaped n electrode18 made of Ti/Al is formed on the surface of the n-side contact layer 4.After masking part of the n electrode 18 and then the p electrode 20 andmulti-layer dielectric films of SiO₂ and TiO₂ are formed, lead-out (pad)electrodes made of Ni—Ti—Au (1000 Å-1000 Å-8000 Å) are formed on the nelectrode 18 and the p electrode 20. Width of the active layer 10 inthis case is 200 μm (width in the direction perpendicular to thedirection of the resonator), and multi-layer dielectric films of SiO₂and TiO₂ are formed also on the resonator surface (on the reflectorside). After forming the n electrode 18 and the p electrode 20, thewafer is divided into bars along the M plane of the nitridesemiconductor (M plane (11-00) of GaN, etc.) in the directionperpendicular to the stripe-shaped electrode, and the bars are furtherdivided into chips, thereby producing the laser devices. Length of theresonator is 650 μm in this case.

The laser device fabricated as described above has threshold current of35 mA, Vf of 3.8 V, Eta of 1.3 W/A, θ (∥) of 8 deg, and θ(⊥) of 22 deg.The device demonstrates good characteristics without kink occurringuntil the output power reaches 80 mW.

Example 2

In Example 2, a nitride semiconductor laser device having the structureshown in FIG. 6 is fabricated. While the first semiconductor layer 22,the second semiconductor layer 24 and the current blocking layer 30 areformed in the p-side optical guide layer 12 in Example 1; these layersare formed in the n-side optical guide layer 8 in this example. Afterforming the first n-side optical guide layer 8 a with a thickness of0.075 μm, the first semiconductor layer 22, the second semiconductorlayer 24 and the current blocking layer 30 are formed and thereafter thesecond n-side optical guide layer 8 b is formed with a thickness of0.075 μm. The laser device thus fabricated is a pulsation laser havingthreshold current of 45 mA, operating frequency of 2 GHz and RIN of −130dB/Hz.

Example 3

In Example 3, a gallium nitride semiconductor laser having the structureshown in FIG. 7 is fabricated. Layers up to the first p-side opticalguide layer 12 a are formed similarly to Example 1.

(Growth Base Layer 26)

With the temperature lowered to 800° C., the growth base layer 26 madeof In_(0.1)Ga_(0.9)N is formed with a thickness of 50 Å by using TMI,TMG and ammonia as the stock material gas.

(Current Blocking Layer 30)

Then the temperature is set to 450° C., and the current blocking layer30 made of Al_(0.9)Ga_(0.1)N is formed with a thickness of 50 Å by usingTMA and ammonia as the stock material gas. The wafer having the layersdescribed above formed thereon is taken out of the furnace of thereaction furnace of the MOCVD apparatus and is processed to form thestripe-shaped window 32 by etching the Al_(0.9)Ga_(0.1)N film 30 with analkaline developer solution similarly to Example 1. Since the growthbase layer 26 made of In_(0.1)Ga_(0.9)N does not dissolve in thealkaline solution, the growth base layer 26 functions as an etchingstopper layer against etching of the Al_(0.9)Ga_(0.1)N layer 30.

Then the wafer is put into the reaction furnace of the MOCVD apparatusof which temperature is set to 1000° C. and hydrogen gas, which is areducing gas, is blown onto the surface so as to carry out etch-back.Since the growth base layer 26 made of InGaN contains InN of lowdecomposition temperature in the mixed crystal thereof and has beengrown at a low temperature, this layer decomposes easily when exposed toreducing atmosphere at a high temperature. That is, the growth baselayer 26 made of In_(0.1)Ga_(0.9)N is removed more easily by etch-backthan the first p-side optical guide layer 12 a grown from GaN at a hightemperature of 1000° C. and the current blocking layer 30 made of AlNthat has high decomposition temperature. Therefore, only the growth baselayer 26 can be selectively removed by properly selecting the conditionsof the etch-back process.

The gallium nitride semiconductor laser is fabricated similarly toExample 1 after forming the second p-side optical guide layer 12 b.

The laser device fabricated as described above has oscillationwavelength of 405 nm, threshold current of 40 mA, Vf of 3.6 V, Eta of1.2 W/A, θ (∥) of 7 deg, and θ (⊥) of 20 deg. The device demonstratesgood characteristics without kink occurring until the output powerreaches 80 mW.

Example 4

In Example 4, a gallium nitride semiconductor laser having the structureshown in FIG. 9 is fabricated. While the growth base layer 26 and thecurrent blocking layer 30 are formed in the p-side optical guide layer12 in Example 3, these layers are formed in the n-side optical guidelayer 8 in this example. After forming the first n-side optical guidelayer 8 a with a thickness of 0.075 μm, the growth base layer 26 and thecurrent blocking layer 30 are formed and thereafter the second n-sideoptical guide layer 8 b is formed with a thickness of 0.075 μm. Thelaser device thus fabricated has threshold current of 40 mA, Vf of 3.9V, Eta of 1.2 W/A, θ (∥) of 7 deg, and θ (⊥) of 24 deg, and demonstratesgood characteristics without kink occurring until the output powerreaches 80 mW.

Example 5

In Example 5, a multi-stripe laser having multiple stripe structure isfabricated. The process is basically the same as that of Example 1,except for following. In this example, after forming the devicestructure up to the first p-side optical guide layer using a GaNsubstrate, the first semiconductor layer made of Al_(0.1)Ga_(0.9)N isgrown with a thickness of 200 Å, the second semiconductor layer made ofGaN is grown with a thickness of 100 Å, the current blocking layer madeof Al_(0.95)In_(0.01)Ga_(0.04)N is grown with a thickness of 200 Å andthen the window was formed. Four windows each 2 μm wide were formed at20 μm intervals. Then the second p-side optical guide layer was re-grownand the remaining device layers were formed. The multi-stripe laserhaving four laser elements of ridge width 2 μm formed in parallel asdescribed above demonstrated good characteristics with threshold currentof 100 mA, Eta of 1.6 W/A and Po of 200 mW.

Example 6

In Example 6, a gallium nitride semiconductor laser device having thestructure shown in FIG. 10 is fabricated. Instead of the firstsemiconductor layer 22, the second semiconductor layer 24 and thecurrent blocking layer 30 of Example 1, only the current blocking layer30 is formed under the conditions described below. With other respects,the process is similar to that of Example 1.

(Current Blocking Layer 30)

With the temperature set to 500° C., the current blocking layer 30 madeof AlN is formed with a thickness of 300 Å by using TMA and ammonia asthe stock material gas. The wafer is then taken out of the reactionfurnace of the MOCVD apparatus and is processed to form thestripe-shaped window 32 as follows. First, the current blocking layer 30is coated with a photo resist over substantially the entire surfacethereof. After the surface is exposed to light in the pattern of thewindow 32, the pattern is developed using TMAH that is an alkalinesolution. Developing process is carried out for two minutes with theTMAH solution of 2.38% maintained at 23° C. Since the AlN layer 30 thathas low crystallinity dissolves in the alkaline developer solution, theportion of the AlN layer 30 located above the window 32 is etched andremoved during the developing process. On the other hand, the portion ofthe AlN layer that makes contact with the first p-type optical guidelayer which is the base layer formed below the current blocking layerhas good crystallinity and does not dissolve under the conditionsdescribed above, and therefore a portion thereof about 80 Å remains inthe window (FIG. 12B).

Then the wafer is put into the reaction furnace of the MOCVD apparatusof which temperature is set to 1000° C. and hydrogen gas, which is areducing gas, is blown onto the surface for about 10 minutes so as tocarry out etch-back. The remaining film portion 30 b that remains in thewindow 32 functions as an etching stopper layer against etch-back. Afterthis etch-back, the remaining film portion 30 b becomes 70 Å inthickness.

The laser device fabricated as described above has threshold current of35 mA, Vf of 3.8 V, Eta of 1.3 W/A, θ (∥) of 8.5 deg, and θ (⊥) of 22.5deg. The device demonstrates good characteristics without kink occurringuntil the output power reaches 80 mW.

Example 7

In Example 7, a gallium nitride semiconductor laser device having thestructure shown in FIG. 13 is fabricated. While the current blockinglayer 30 is formed in the p-side optical guide layer 12 in Example 6,the current blocking layer 30 is formed in the n-side optical guidelayer 8 in this example. After forming the first n-side optical guidelayer 8 a with a thickness of 0.075 μm, the current blocking layer 30made of AlN is formed. After forming the window 32 so that the remainingfilm portion about 30 Å thick remains, the second n-side optical guidelayer 8 b is formed with a thickness of 0.075 μm. The laser device thusfabricated is a pulsation laser that has threshold current of 40 mA, Vfof 4.0 V, Eta of 1.1 W/A, θ (∥) of 7.5 deg, and θ (⊥) of 20 deg, anddemonstrates good characteristics without kink occurring until theoutput power reaches 80 mW.

Example 8

In Example 8, a multi-stripe laser having multiple stripe structure isfabricated. The process is basically the same as that of Example 6except for the process described below. In this example, a GaN substrateis used and the n electrode is led out of the back surface of the GaNsubstrate. After forming the device structure up to the first p-sideoptical guide layer, the first semiconductor layer made ofAl_(0.1)Ga_(0.9)N is grown with a thickness of 200 Å, the secondsemiconductor layer made of GaN is grown with a thickness of 100 Å, thecurrent blocking layer made of Al_(0.95)In_(0.01)Ga_(0.04)N is grownwith a thickness of 200 Å and then the window was formed. Four windowseach 2 μm wide were formed at 20 μm intervals. Then the second p-sideoptical guide layer was re-grown and the remaining device layers wereformed. The n electrode made of Ti/Al (100 Å/5000 Å) was formed on theback surface after polishing. The multi-stripe laser having four laserelements of ridge width 2 μm formed in parallel as described abovedemonstrated good characteristics with threshold current of 100 mA, Etaof 1.6 W/A and Po of 200 mW.

While the present invention has been described in full detail inconjunction with the preferred embodiments while making reference to theaccompanying drawings, it will be apparent for those skilled in the artthat various alterations and modifications can be made. Such alterationsand modifications should be understood as included in the presentinvention unless they deviate from the scope of the invention that isdefined by the appended.

1. A nitride semiconductor laser comprising: a laminate including ann-side semiconductor layer; an active layer; and a p-side semiconductorlayer, wherein one of said n-side semiconductor layer and said p-sidesemiconductor layer includes a current blocking layer having astripe-shaped window formed therein to pass current flow, wherein saidstripe-shaped window is formed in said current blocking layer such thata residual film portion is formed in said current blocking layer andsuch that said residual film portion is formed along said stripe-shapedwindow of said current blocking layer, wherein said residual filmportion is formed in said current blocking layer, such that a part ofsaid current blocking layer that makes contact with a layer lyingthereunder remains in an entirety of said striped-shaped window, whereina thickness of said residual film portion formed in said currentblocking layer is smaller than a thickness of other portions of saidcurrent blocking layer, such that current is injected into said activelayer through said residual film portion, wherein the thickness of saidresidual film portion is not less than 10 Å and the thickness of saidresidual film portion is smaller than 100 Å, and wherein a totalthickness of said current blocking layer is not less than 100 Å and thetotal thickness of said current blocking layer is not more than 800 Å.2. The nitride semiconductor laser according to claim 1, wherein saidcurrent blocking layer, except for said residual film portion, is formedapart from end faces of said laminate.
 3. The nitride semiconductorlaser according to claim 1, wherein said current blocking layer isformed in said p-side semiconductor layer.
 4. The nitride semiconductorlaser according to claim 1, wherein a semiconductor layer that fillssaid striped-shaped window of said current blocking layer is made of anitride semiconductor that is without Al.
 5. The nitride semiconductorlaser according to claim 1, wherein a semiconductor layer that fillssaid striped-shaped window of said current blocking layer serves as alight-guiding layer.
 6. The nitride semiconductor laser according toclaim 1, wherein a dislocation density of a region over saidstriped-shaped window of said current blocking layer is smaller than thedislocation density of a region over the other portions of said currentblocking layer.
 7. The nitride semiconductor laser according to claim 1,wherein said current blocking layer comprises AlN.
 8. The nitridesemiconductor laser according to claim 1, wherein a p-side ohmicelectrode is formed on said p-side semiconductor layer, such that awidth of said p-side ohmic electrode is larger than a width of saidstriped-shaped window of said current blocking layer and is smaller thana total width of said current blocking layer.
 9. The nitridesemiconductor laser according to claim 8, wherein a length of saidp-side ohmic electrode in a direction parallel to light propagation isshorter than a length of said current blocking layer.
 10. A nitridesemiconductor laser comprising: a laminate including an n-sidesemiconductor layer; an active layer; and a p-side semiconductor layer,wherein one of said n-side semiconductor layer and said p-sidesemiconductor layer includes a current blocking layer having astripe-shaped window formed therein to pass current flow, wherein saidstripe-shaped window is formed in said current blocking layer such thata residual film portion is formed in said current blocking layer andsuch that said residual film portion is formed along said stripe-shapedwindow of said current blocking layer, wherein said residual filmportion is formed in said current blocking layer, such that a part ofsaid current blocking layer that makes contact with a layer lyingthereunder remains in an entirety of said striped-shaped window, whereina thickness of said residual film portion formed in said currentblocking layer is smaller than a thickness of other portions of saidcurrent blocking layer, such that current is injected into said activelayer through said residual film portion, wherein a semiconductor layerthat fills said striped-shaped window of said current blocking layer ismade of a nitride semiconductor that is without Al, and wherein saidcurrent blocking layer comprises AlN.
 11. The nitride semiconductorlaser according to claim 10, wherein the thickness of said residual filmportion is not less than 10 Å and the thickness of said residual filmportion is smaller than 100 Å.
 12. The nitride semiconductor laseraccording to claim 10, wherein a total thickness of said currentblocking layer is not less than 100 Å and the total thickness of saidcurrent blocking layer is not more than 800 Å.
 13. The nitridesemiconductor laser according to claim 10, wherein said current blockinglayer, except for said residual film portion, is formed apart from endfaces of said laminate.
 14. The nitride semiconductor laser according toclaim 10, wherein said current blocking layer is formed in said p-sidesemiconductor layer.
 15. The nitride semiconductor laser according toclaim 10, wherein a semiconductor layer that fills said striped-shapedwindow of said current blocking layer serves as a light-guiding layer.16. The nitride semiconductor laser according to claim 10, wherein adislocation density of a region over said striped-shaped window of saidcurrent blocking layer is smaller than the dislocation density of aregion over the other portions of said current blocking layer.